To determine the number of DAZ gene clusters in the Y-bearing spermatozoa of patients who underwent intracytoplasmic sperm injection (ICSI) and to compare the outcome with the number of clusters found in the spermatozoa of normospermic men. Prospective study. Academic hospital.Forty-seven patients with impaired spermatogenesis who were attending our clinic for ICSI and 56 semen donors. Peripheral blood was drawn to obtain somatic DNA for polymerase chain reaction (PCR) analysis and leukocytes for karyotyping and FISH analysis. Three-color FISH was performed on the spermatozoa remaining after ICSI and on the spermatozoa of semen donors to determine the presence of the X and Y chromosome as well as the number of DAZ gene clusters. Number of DAZ gene clusters in Y-bearing spermatozoa. Five patients had only one DAZ gene cluster, one patient had a complete AZFc deletion, and one patient had three clusters on average. One of the semen donors also showed three DAZ gene clusters in his Y-bearing spermatozoa. None of the semen donors had only one DAZ gene cluster. Besides complete AZFc deletions, partial deletions are also associated with impaired spermatogenesis. As a result, these partial deletions that are not recognized by routine PCR are reintroduced into the population by the ICSI technique.