In a 46XY normotensive infant with micropenis, perineoscrotal hypospadias and a bifid scrotum with histologically normal testes, 170H-deficiency was diagnosed from increased serum corticosterone (B) and progesterone (P) plus inadequate cortisol (F), testosterone (T) and DHA responses. Urine B metabolites were raised 100-fold; the 5α/5β-reduced metabolite ratio was normal before and after cortisol loading. Plasma Na, K, aldosterone and renin were normal.Genital skin fibroblast T receptors were normal in vitro (23 fmol/ mg) but surprisingly 5α-reductase activity was reduced (0.4 pM/μg DNA/hr), although Km for T (0.12 μM) and NADPH (170 μM) were normal. T therapy corrected the micropenis but not the 5α-reductase deficiency. Family study suggested reduced 170H activity in the father and reduced 5α-reductase activity in the mother. Although the data suggest this boy may be a double heterozygote, a more direct link between 170H-deficiency and target cell 5α-reductase cannot be excluded.