Deficiency of 5alpha-reductase type 2 activity causes deficient masculinization of 46,XY individuals caused by a lack of dihydrotestosterone. At puberty, virilization is often observed. A precise diagnosis with correct gender assignment at an early age is very important. Recently, the molecular basis of the enzyme defect was discovered; however, only a few cases of 5alpha-reductase deficiency with a complete molecular genetic analysis have been published. We report on a Turkish patient clinically classified with steroid 5alpha-reductase deficiency (SRD) type 3b (karyotype 46,XY) who was raised as a girl and presented to us at the age of 14 years because the male phenotype had become predominant at puberty. Endocrinological investigations revealed an elevated serum testosterone/dihydrotestosterone ratio (17.3, normal: <16). PCR-SSCP analyses detected a deletion of methionine on exon 3 of the 5alpha-reductase type 2 gene.