Male pseudo hermaphroditism caused by steroid 5 alpha reductase deficiency is a rare autosomal recessive disorder. This enzyme catalyses the conversion of testosterone to dihydrotestosterone (DHT) in genital tissue. The potent androgen DHT is required for full masculinization of the external genitalia hence, masculinization defects of varying degree result from diminished DHT formation. We report a Tunisian patient who was raised as a girl, presented to us at the age of 15, because male phenotype had become predominant at puberty. Endocrinological investigations revealed an in crease in the ratio serum testosterone/DHT = 17. Treatment with dihydrotestosterone and surgical correction, after psychological evaluation permitted the change of gender identity to male.