A series of 10 young sterile men with acephalic spermatozoa or abnormal head-mid-piece attachments is presented. Nine of these patients had 75-100% spermatozoa with minute cephalic ends and 0-25% abnormal head-middle piece attachments. Loose heads ranged between 0-35 for each 100 spermatozoa and normal forms were rare. Two patients were brothers. On ultrastructural examination, the head was generally absent and the middle piece was covered by the plasma membrane. When present, heads implanted at abnormal angles on the middle piece. A testicular biopsy showed abnormal spermiogenesis. The implantation fossa was absent and the flagellar anlage developed independently from the nucleus, resulting in abnormal head-middle piece connections. In one patient azoospermia was induced with testosterone to attempt to increase the normal sperm clone during the rebound phenomenon, but all newly formed spermatozoa were acephalic. In another patient with high numbers of defective head-mid-piece connections, microinjections of spermatozoa resulted in four fertilized oocytes, but syngamy and cleavage did not take place, suggesting an abnormal function of the centrioles. The findings indicate that acephalic spermatozoa arise in the testis as the result of an abnormal neck development during spermiogenesis. The familial incidence and the typical phenotype strongly suggest a genetic origin of the syndrome.