The 84-kDa hormone-sensitive lipase (gene designation Lipe; EC 3.1.1.3) is a cholesterol esterase and triglyceride hydrolase that functions in the release of fatty acids from adipocytes. The role of hormone-sensitive lipase in other tissues such as the testis, where a specific 120-kDa testis-specific isoform is expressed, is unknown. To study this, we examined the fertility and testicular histology of gene-targeted hormone-sensitive lipase-deficient mice. Homozygous hormone-sensitive lipase-deficient male mice are infertile and have decreased testis weights; female homozygotes are fertile. Testicular abnormalities, detected at the light and electron microscopic levels, included the presence of multinucleated round and elongating spermatids, vacuolization of the seminiferous epithelium, asynchronization of the spermatogenic cycle, sloughing of postmeiotic germ cells from the seminiferous epithelium into the lumen, and a marked reduction in the numbers of late spermatids. Extensive nuclear head deformation was noted in late spermatids as well as the sharing of a common acrosome in multinucleated cells. In some multinucleated cells, nuclei were separated from their acrosomes, with the acrosomes remaining attached to areas of ectoplasmic specializations, suggesting defects in intercellular cytoplasmic bridge integrity. Although the lumen of the epididymis was essentially devoid of spermatozoa and filled instead with spherical degenerating cells, the epididymal epithelial cells appeared normal. The few late spermatids present in the epididymis were abnormal. There was no morphological evidence, as judged by the absence of lipid droplets of triacylglycerol or cholesteryl ester accumulation in the testis. Together, the data suggest that hormone-sensitive lipase deficiency results in abnormalities in spermiogenesis that are incompatible with normal fertility. We speculate that a metabolite downstream from the hormone-sensitive lipase reaction may be essential for membrane stabilization and integrity in the seminiferous epithelium and, in particular, may play an important role in the maintenance of intercellular cytoplasmic bridges between postmeiotic germ cells.