The birth of a child with ambiguous genitalia represents a very stressing situation for the family, and afterwards has great social and psychological repercussion for the patient itself. Until now, the sex assignation is being done according to the phenotype. Now, with the molecular diagnosis of the genes that play a role in the sexual development, the assignation must also take into account the prognosis of response to androgens. The aim of this work is to review the 40 male pseudohermaphroditism cases controlled in our hospital and the genetic molecular diagnosis done in 19 cases, thus obtaining the certainty diagnosis. In 15 patients the mutations were located in the AR gene (androgen receptor). In 2 cases the mutation affected the SRD5A2 gene (deficiency of 5a-reductase) and in the other 2 cases it affected the HSD17BIII gene (deficiency of 17-ketoreductase). If the mutations affect the AR gene they must be assigned to the feminine sex, because of the impossibility of virilisation at puberty (lack of response to androgens). If the mutations are located in the other 2 genes they can be assigned to the masculine sex, since in puberty, they will present good response to androgens and will virilize. The molecular diagnosis offers us also the possibility to establish a prenatal diagnosis and to offer genetic advice.