Abstract Neonatal alloimmune thrombocytopenia (NAIT) is caused by maternal antibodies directed against fetal platelet antigens inherited from the father. Approximately 80% of cases of NAIT are attributed to antibodies against human platelet antigen (HPA)-1a and less commonly to antibodies against HPA-5b. We present two cases of possible NAIT attributed to anti-HPA-5b that were identified in sisters requiring concurrent management. Patient #1 is a 32-year-old G9P4044 woman with one pregnancy complicated by intrauterine growth retardation and partial placental abruption, another pregnancy complicated by fetal anomaly (intracerebral hemorrhage) status post termination of pregnancy at 23 weeks’ gestation and three miscarriages (one with chromosomal abnormality). The patient is homozygous for HPA-1a and HPA-5a, and the partner is homozygous for HPA-5b. HPA-5b antibodies and HLA antibodies were identified in the patient’s serum. Deemed at high risk for subsequent NAIT, she was started on intravenous immunoglobulin twice weekly and steroids during her second trimester until delivery. The patient underwent weekly fetal sonograms during pregnancy which were normal. The patient’s platelet counts were stable. The patient refused caesarean delivery. At 38 weeks’ gestation, the patient gave birth to a healthy female via normal spontaneous vaginal delivery. HPA-5b negative platelets were procured from our blood supplier, but neither the mother nor neonate required transfusion. The neonate showed no signs of bleeding and had normal platelet counts. Ultrasound imaging of the head was unremarkable. The patient and neonate were discharged home. Patient #2 is a 33-year-old G13P6156 woman with a prior pregnancy complicated by stillbirth at 32 weeks’ gestation. Considering her personal and family history, HPA testing was performed and HPA-5b antibodies were identified in the patient’s serum. The patient’s partner is heterozygous for HPA-5b. Crossmatched platelets were procured from our blood supplier. The patient refused caesarean delivery. The patient gave birth at 37 weeks’ gestation to a healthy male via normal spontaneous vaginal delivery. The fetus had mild thrombocytopenia (107 x 103/uL), with no bleeding or bruising identified. Neither the mother nor neonate required transfusion. The patient and neonate were discharged home. Ultrasound imaging of the head was unremarkable, and his thrombocytopenia resolved. While some studies postulate that anti-HPA-5b identification may be coincidental rather contributory to the disease process, these cases re-emphasize the importance of obtaining pregnancy, neonatal and family history to best risk stratify patients and determine management from both the clinical and transfusion medicine perspectives. If transfusion is needed, transfusion options include HPA negative platelets, crossmatched platelets and random donor platelets depending on clinical urgency and platelet availability.