A woman with strong history of endometriosis underwent amniocentesis because of age (37). Amniotic cell cultures done at the Birth Defects Institute, NYS Dept. of Health, showed 30% trisomy 20 cells in the first tap and 13% in the second tap (Harrison et al., Am J Human Gen 32:71A, 1980). At term an 8 lb 10 oz infant girl was delivered. She was clinically and neurologically normal except for a 1 cm area of denuded skin to the left of the midline in the occipital area, reminiscent of the scalp lesion of trisomy 13. By 6 weeks of age it had developed a raised cicatrix, as in trisomy 13 during “healing”. Blood karyotype was 46,XX. Developmental milestones have remained normal to present age 16 months. Four normal live births have been previously reported after trisomy 20 mosaicism was detected in amniocytes (Am J Med Gen 2:365, 1978; Lancet 1:1089, 1980). At least 8 such pregnancies have been terminated, with 5 fetuses described as normal, 2 as dysmorphic and one not examined. Only in 2 cases have trisomy 20 cells been found in fetal tissues. Trisomy 20 mosaicism remains a troublesome finding in amniotic cell cultures. Whether the cells are due to cultural artifact (Harrison et al., ibid.) or are derived from early trophoblast (Warburton) or fetal renal tissue (Boué) is still an open question. The “trisomy 13” scalp lesion in the present patient seems hardly coincidental.