Patients with full trisomy 15 or mosaic trisomy 15 have been rarely reported. Many literature articles describe identifying trisomy 15 in products of conception, deeming it one of the more common causes of miscarriages related to chromosome anomalies (∼7%). However, the liveborn incidence of mosaic trisomy 15 has been described in only ten cases in the literature. Patients with mosaic trisomy 15 have been described to have intrauterine growth restriction, congenital anomalies of the heart defects, brain, kidneys, and genitalia, skeletal findings including rib anomalies and abnormal hand posture, dysmorphic craniofacial features, cleft palate, and developmental delay. Unfortunately, many individuals have passed away due to their multiple congenital anomalies within the first year of life. We report here on a 16-month-old female with a diagnosis of mosaic trisomy 15. Our patient is the first child born to her parents. The pregnancy was complicated by gestational diabetes that was diet controlled. She was born at 37 weeks weighing 2.65 kg (24th percentile), was 47 cm long (38th percentile) and had an OFC of 33.5 cm (55th percentile). She required interventions for respiratory distress shortly after birth in the form of CPAP. Echocardiogram was notable for a large ASD, multiple small VSDs, PDA, and a bicuspid aortic valve. She developed worsening peripheral pulmonary hypertension and was transferred to our center for tertiary care. Imaging identified left pleural effusion and she was found to have a chylothorax. Due to the congenital heart defects and chylothorax, Genetics was consulted. We recommended a microarray analysis which identified a mosaic gain of chromosome 15 in 20% of the peripheral blood sample. Chromosome analysis of the PHA-stimulated blood didn’t reveal gain of chromosome 15. We also completed methylation and copy number analysis for Prader-Willi/Angelman syndrome critical region. This result was reported as normal, although showed a trend for chromosome 15 copy number gain and that the extra copy of chromosome 15 was maternally derived. This is consistent with a most likely mechanism of trisomy 15 conception due to maternal non-disjunction followed by a trisomy rescue event post-zygotically. During a cardiac procedure, a small sample of cardiac tissue was obtained. A microarray analysis completed on the tissue sample was notable for a mosaic, 75-80% gain of chromosome 15. She was hospitalized for the first five months of her life during which she required pulmonary artery banding, coarctation repair, PDA ligation, and stent placement/balloon dilation for pulmonary vein stenosis. Escalating needs for respiratory support ended in intubation. She failed extubation trials and was found to have subglottic stenosis. Balloon dilation, steroid injections, and a cricoid split failed and a tracheostomy was placed. Prior to discharge, a g-tube was also placed due to poor coordination and inability to feed safely. Over the next several months, she had multiple readmissions for complications of her heart disease and increasing respiratory needs. She was admitted to an outside hospital for repeat balloon dilation, VSD closure, partial ASD closure, pulmonary artery de-banding, and stent extraction with sutureless pulmonary vein repair. She has had multiple cardiac catheterizations due to her recurrent/residual left sided pulmonary vein stenosis and elevated pulmonary vascular resistance. She has developed horizontal nystagmus, hyperopia and pseudoesotropia. There are no clinical concerns for hearing loss with a normal newborn hearing screen and an auditory brainstem response study planned. She continues to require oxygen support via ventilation. She is g-tube dependent and trialing tastes by mouth. Her physical examination is notable for hypertelorism, epicanthal folds, depressed nasal bridge, anteverted short nares, and small and wide spaced teeth. Her weight is at the 4th centile, her length is at the 50th centile for a 9-month-old, and her head circumference is at the 50th percentile for a 7-month-old. An abdominal ultrasound noted increased echogenicity of the kidneys. A brain MRI was notable for mild to moderate prominence of the subarachnoid spaces. At 16 months old, she can sit without support and is pulling to a stand independently. She is rolling and army crawling. She is reaching for and holding objects, transferring them from hand to hand. She is extremely social and is frequently smiling. A tracheostomy remains in place and limits her vocal abilities. She seems to understand more than 25 words and will choose words from her communication board including “yes”, “no”, “sleep” and “eat”. She receives multiple therapy services. Cases of liveborn mosaic trisomy 15 are rare, with most children passing away in infancy due to congenital anomalies. Our patient is a 16-month-old female with a complex history who continues to defy the odds. She presented with a chylothorax, which has never been reported in this condition. She also has significant pulmonary vein stenosis and pulmonary vascular resistance, not previously reported. Despite these complications, she is continuing to make developmental progress. Our patient is one of the few liveborn children with mosaic trisomy 15 to survive past the first year of life.