This 3 mos old boy was found to have a karyotype of 46,XY,-18, +t(11;18)(q23;q23). His mother had a balanced reciprocal trans-location 46,XX,t(11;18)(23;23). He had dysmorphic, low-set ears, asymmetrical facies, epicanthus and high-arched palate. Lips had a “fish-mouth” appearance. There were: widely-spaced nipples, narrow chest, umbilical and bilateral inguinal hernias, micropenis, normal scrotum and generalized hypotonia. Autopsy at age 13 mos showed trigonocephaly, VSD, pulmonary a. stenosis and other cardiac anomalies, and histologically normal testes in canals. Clavicles and other bones were normal by x-ray. There have been 4 males previously described with micropenis and partial trisomy 11q syndrome (Pihko et al. Hum Genet 58:129, 1981). Excluded are trisomies due to t(11;22) which may be a separate syndrome (DeFrance et al. Clin Genet 25:295, 1984). Only one other case has been reported with t(11;18), but the receptor region was 18p rather than 18q, and the 11q breakpoint was much more proximal (q14). Etiology of micropenis in partial trisomy 11q syndrome may be associated with defects in midline brain structures, e.g. corpus callosum (cf. micropenis in septo-optic dysplasia syndrome where septum pellucidum is absent). This study supports the hypothesis that most signs of partial trisomy 11q syndrome are caused by trisomy of regions distal to 11q23.