During epididymal sperm maturation, the lipid content of the sperm membrane is modified, which facilitates sperm motility and fertility. However, little is known about the mechanisms regulating the maturation process. By generating a conditional knockout (cKO) of Dicer1 in the proximal part of the mouse epididymis, we studied the role of RNA interference in epididymal functions. The Dicer1 cKO epididymis displayed an altered lipid homeostasis associated with a 0.6-fold reduction in the expression of the gene elongation of very long chain fatty acids-like 2, an enzyme needed for production of long-chain polyunsaturated fatty acids (PUFAs). Furthermore, the expression of several factors involved in cholesterol synthesis was up-regulated. Accordingly, the Dicer1 cKO sperm membrane showed a 0.7-fold decrease in long-chain PUFAs, whereas the amount of cholesterol in acrosome-reacted sperm displayed a 1.7-fold increase. The increased cholesterol:PUFA ratio of the sperm membrane caused breakage of the neck and acrosome region and immotility of sperm. Dicer1 cKO mice sperm also displayed reduced ability to bind to and fertilize the oocyte in vitro. This study thus shows that Dicer1 is critical for lipid synthesis in the epididymis, which directly affects sperm membrane integrity and male fertility.-Bjorkgren, I., Gylling, H., Turunen, H., Huhtaniemi, I., Strauss, L., Poutanen, M., Sipila, P. Imbalanced lipid homeostasis in the conditional Dicer1 knockout mouse epididymis causes instability of the sperm membrane.