OBJECTIVE To determine whether vaccination with a monovalent vaccine against Leptospira borgpetersenii serovar Hardjo strain hardjobovis would improve reproductive efficiency in Holstein cattle in a commercial dairy setting. METHODS Randomized controlled trial. METHODS 1,894 Holstein cows and heifers from a Central California dairy. METHODS Cattle were assigned to undergo SC administration of a monovalent vaccine against Leptospira borgpetersenii serovar Hardjo strain hardjobovis (n = 986) or a placebo (lactated Ringer's solution; 908). At the end of their lactation period, cows received 2 doses of the vaccine or placebo, 28 to 35 days apart, with the initial dose administered in conjunction with oxytetracycline. Heifers received the same treatments, with the second dose administered at least 2 weeks before their entrance into the heifer breeding pen. Urine and blood samples were collected from randomly selected cattle immediately before and 1 year after the trial began and submitted for fluorescent antibody and microscopic agglutination testing to identify any infecting Leptospira serovar. RESULTS The initial herd prevalence of active infection with strain hardjobovis was 13% (6/46 tested cattle), followed by 15% (6/40) 1 year after the trial began. The odds of heifers conceiving over the period at risk for conception, regardless of vaccination, was approximately 2.8 times as high as for primiparous and pluriparous cows. Survival analysis of days from parturition to conception revealed that the vaccine protocol had no effect on the probability of conception between the vaccinated and control groups. The vaccine protocol had no impact on pregnancy loss. CONCLUSIONS The evaluated vaccination protocol against Leptospira strain hardjobovis was not effective in improving reproductive efficiency in commercial Holstein dairy cows or in decreasing urine shedding of leptospires.