Sperm chromosome studies have been performed in 70 normal males. The incidence of aneuploidy in this group is approximately 3-4%, and that of structural anomalies close to 5%. In carriers of reciprocal or Robertsonian translocations, the results are extremely variable, with percentages of unbalanced sperm from 8 to 87%. No unbalanced spermatozoa have been observed in patients with pericentric or paracentric inversions. In cancer patients treated with radio and/or chemotherapy, the incidence of chromosome abnormalities is much higher, and significantly different from that found in controls.