For nonobstructive azoospermic (NOA) patients with a normal karyotype or for Klinefelter syndrome (47,XXY) patients, intracytoplasmic sperm injection is associated with an increased aneuploidy risk in offspring. We examined testicular cells from patients with different azoospermia etiologies to determine the origin of the aneuploid spermatozoa. The incidence of chromosome abnormalities was investigated in all types of azoospermia. Four study subgroups were constituted: Klinefelter patients (group 1), NOA patients with spermatogenesis failure but a normal karyotype (group 2), obstructive azoospermic patients with normal spermatogenesis (group 3), and control patients with normal sperm (group 4). The pachytene stage (in the three azoospermic groups) and postmeiotic cells (in all groups) were analyzed with fluorescence in situ hybridization. No aneuploid pachytene spermatocytes were observed. Postmeiotic aneuploidy rates were higher in the two groups with spermatogenesis failure (5.3% and 4.0% for groups 1 and 2, respectively) than in patients with normal spermatogenesis (0.6% for group 3 and group 4). Whatever the etiology of the azoospermia, the spermatozoa originated from euploid pachytene spermatocytes. These results strengthen the hypothesis whereby sperm aneuploidy in both Klinefelter patients and NOA patients with a normal karyotype results from meiotic abnormalities and not from aneuploid spermatocytes. The fact that sperm aneuploidy was more frequent when spermatogenesis was altered suggests a deleterious testicular environment. The study results also provide arguments for offering preimplantation genetic diagnosis or prenatal diagnosis when a pregnancy occurs for fathers with NOA (whatever the karyotype).