Klinefelter syndrome is caused by a chromosomal anomaly which implies a partial or total duplication of chromosome X in presence of chromosome Y. It is reported the case of a sixmonth old child who was the second child of a non-related couple. The physical exam revealed a general hypotony, hipertelorism, microretrognatia, piebot, hypoplastic penis and imperforete anus. Chromosomal preparations were obtained from 72 hour-lymphocyte cultures which showed the presence of fuor chromosomes X and 1chromosome Y. The karyotype was 49, XXXXY in pure line