Leptospirosis isone of the most widespread zoonotic infections known and is caused by a diverse group of Leptospira species. Until recently, attempts to demonstrate reproducible experimental infection of golden Syrian hamsters with L. borgpetersenii serovar Hardjo were unsuccessful. We showed that serovar Hardjo can establish either a chronic infection with few outward signs of disease or cause an acute, debilitating and lethal infection in hamsters, depending upon the strain used. The virulent strain induced neutrophilia and formed neutrophil aggregates. Neutrophil aggregation may be a unique feature of virulent serovar Hardjo; aggregates were not detected in animals infected with either L. interrogans serovar Pomona or L. kirschneri serovar Grippotyphosa. Circulating aggregates may form microemboli and contribute to hemorrhage in various tissues, especially lung. Serovar Hardjo strains colonized brain in both acute and chronic infection models without apparent induction of inflammatory cell infiltration. Although there is limited evidence in the literature that Leptospira can colonize the brain, the biological consequences are unknown. Continued use of this infection model should help elucidate Leptospira invasion mechanisms and lead to a better understanding of the pathological consequences of infection.