Technical Abstract: Chronic infection of cattle with Leptospira borgpetersenii serovar Hardjo reduces animal production through reproductive failure and presents a persistent health threat to workers in the animal industry. Cattle are maintenance hosts for serovar Hardjo and development of a protective vaccine has been problematic. Recent studies indicated that a monovalent vaccine containing killed-whole cells of L. borgpetersenii serovar Hardjo provided short-term protection against renal colonization and urinary shedding of bacteria. A key feature of this vaccine appears to be its ability to induce proliferation of gamma delta-T cells and gamma interferon in response to serovar Hardjo antigens. Questions still remain whether this vaccine provides sterile immunity, i.e. blocks renal colonization and urinary shedding of bacteria, and whether this vaccine induces enduring immunity beyond 4 months following vaccination. In this study, long (12 month) and short-term (3 months) protection induced through use of a commercial monovalent serovar Hardjo vaccine was tested. Using PCR-based methods to detect bacteria, we show that renal colonization and urinary shedding are sharply reduced in vaccinated animals, but in most animals sterile immunity was not achieved. Additionally, animals challenged 1 yr following the last immunization showed higher levels of renal infection than those challenged at 3 months following vaccination. These results suggest the possible need for increased frequency of immunization and antibiotic treatment to reduce renal colonization and urinary shedding of bacteria, and suggest that further improvements to existing serovar Hardjo vaccines are needed.