The diagnosis of 5 alpha-reductase deficiency in infancy is reported for the first time in three male pseudohermaphrodites from the Dominican Republic. Basal plasma testosterone to dihydrotestosterone ratios were significantly elevated in two of the three affected infants, and increased markedly in all three infants after administration of hCG. Since urinary etiocholanolone to androsterone ratios could not be determined accurately in this age group, the diagnosis of 5 alpha-reductase deficiency was confirmed by the finding of elevated urinary tetrahydrocortisol (THF) to 5 alpha-tetrahydrocortisol (5 alpha-THF) ratios, as determined by gas chromatography/mass spectrometry, in the affected male infants compared to those in age-matched normal infants. The THF/5 alpha-THF ratios, however, were lower in both the normal children and the affected infants, suggesting increased 5 alpha-reductase activity in infancy. The affected infants had THF/5 alpha-THF ratios comparable to ratios in adult carrier males and significantly lower than ratios in adult homozygotes. Two of the three affected infants are related to the large Dominican kindred we studied previously and initially were raised as females but, after parental counseling, were changed to a male sex of rearing.