Background: Mutations in type II 3ß hydroxysteroid Dehydrogenase (3ßHSD) are found in male children with severe undervirilized genitalia. Mild undervirilization (isolated micropenis or with distal hypospadia) can be associated with a partial deficit in 3ßHSD activity. Aim: To assess the frequency of abnormal adrenal response to ACTH, suggesting a deficit in adrenal enzymatic activity, in children with mild undervirilization. Patients and methods: We studied 26 male children with micropenis, aged one to eight years. Children with evidences of puberal development or in treatment with drugs that affect steroidal metabolism were excluded from the study. Serum levels of androstenedione (A), dehydroepiandrosterone (DHEA), progesterone (P), 17 hydroxyprogesterone (17 P) and the ratios DHEA/A, P/17 P, 17 P/DHEA were measured after an adrenal stimulation with 0.25 mg/m2 intramuscular ACTH. Results: Two children had DHEA y DHEA/A values suggesting a defective 3ßHSD activity. Other two children had high levels of 17 P, suggesting a deficiency of cytochrome p450c21. A CYP 21 gene mutation was found in one of the later children. Conclusions: A low proportion of children with micropenis have a deficient 3ßHSD activity (Rev Méd Chile 2003; 131: 46-54)