Leptin-deficient (ob/ob) male mice are morbidly obese and exhibit impaired reproductive function. The objective of this study was to assess the effect of a leptin deficiency on testicular morphology, germ cell development, apoptotic activity within germ cells, and expression levels of apoptosis-related genes in the testis. Sixteen week-old ob/ob male mice (n ϭ 8) and controls (n ϭ 8) were killed, and their reproductive organs were weighed. Testes were processed for either histomorphological analysis (hematoxylin and eosin [H&E] staining), germ cell apoptosis assessment (deoxy-UTPdigoxigenin nick end labeling [TUNEL] method), or apoptosis-related gene expression analysis (microarray). Cross sections of the testes of leptin-deficient animals showed reduced seminiferous tubule area, fewer pachytene spermatocytes, and fewer tubules exhibiting elongated spermatids/mature spermatozoa. Condensation of germ cell nuclei and Sertoli cell vacuolization were evident in the testes of some ob/ob animals. Overall there was an elevation of apoptotic activity in the germ cells of ob/ob mice, particularly within the pachytene spermatocytes. With microarray technology, we identified 9 proapoptosis-related genes that were expressed at a significantly higher level in the testes of ob/ob mice than in the testes of the controls. Among these were members of the tumor necrosis factor receptor super family 1A and 5 (TNFR1 and 5) and peptidoglycan recognition proteins (associated with the extrinsic apoptotic pathway), and granzymes A and B, growth arrest and DNA damage inducible 45 gamma, sphingosine phosphate lyase 1, and caspase 9 (associated with the intrinsic apoptotic pathway). The results of the current study show that a leptin deficiency in mice is associated with impaired spermatogenesis, increased germ cell apoptosis, and upregulated expression of proapoptotic genes within the testes.