2 related females with 3bOHD from the same village in Eastern Switzerland were studied. Pt.1 is a 25 year old adult, whose earlier findings have been reported (Zachmann et al., J.clin.Endocr. Metab.30, 719, 1970), pt.2 6 wks old. The mother of pt.1 is a first cousin of the maternal grandmother of pt.2. The relation of the fathers could not be established, but they originate from related local families. Urinary steroids were estimated by gas chromatography on a glass capillary column, their identity was confirmed by mass spectrometry. In pt.1, the main D5S were D5pregnenetriol (110 umol/l),17OH-pregnenolone (6.0), and DHEA (5.6). For reasons discussed earlier, there were also large quantities of hepatic 3a-and 3b-pregnanetriol (114.9). Pt.2 excreted less D5pregnenetriol (12.6 umol/l), but large quantities of more unusual D5S such as 16OH-pregnenolone (14.4), 16OH-DHEA syn (159.5) and anti (101.5), 16keto-D5androstenediol(66.5), 3, 16, 17-D5androstenetriol (33.1), D5pregnene-3, 15, 17-triol-20-one(29.2), 16OH-pregnenolone (113.8), D5pregnene-3, 15, 17, 20-tetrol(78.6), and 21OH-pregnenolone(18.4). It is concluded that urinary steroids of newborns with 3bOHD are modified considerably by the 16-hydroxylating activity of the fetal adrenals and that they differ from those of older children or adults with genetically the same defect. Without specific steroid analyses, this may cause diagnostic difficulties. Supported by Swiss National Science Foundation Grant No.3.874.83