The authors define the field of the sexual ambiguities including not only the cases of ambiguousness in the external genitals, but also those cases which present anomalies in the external or internal genitals or in the gonads, with more or less evident characteristics of the other sex, even if there is no doubt as to the sex attribution. On the contrary the cases with only the lack of gonads are excluded. After a brief recall to some aspects of the sexual differentiation, the authors point up the importance of genetic counselling not only to let a precocious sex registration at the General Register Office, but also with the purpose of a genetic prevention and of a quick corrective intervention. At last the authors propose a personal classification of these defects.