The purpose of this study was to review recent cases of leptospirosis seen at referral centers in New York State and to identify differences in clinical or clinicopathologic aspects of the disease among different suspected infecting serogroups. Medical records at the Cornell University Hospital for Animals and the Animal Medical Center in New York City were reviewed to identify dogs diagnosed with leptospirosis from September 1996 to August 2002. Records of 55 dogs met the inclusion criteria for the study. The suspected infecting serogroups included 21 occurrences of Grippotyphosa, 12 of Pomona, 6 of Autumnalis, 5 of Bratislava, 2 of Hardjo, and 1 of Canicola. Five dogs had equal titers to serogroups Grippotyphosa and Pomona, and 3 had equal titers to 2 other serogroups. Common clinical signs included lethargy, anorexia, and vomiting. Common clinicopathologic findings included anemia, thrombocytopenia, azotemia, hyperphosphatemia, high liver enzyme activity, and hyperbilirubinemia. Forty-three of 55 dogs were discharged from the hospital. Serogroup-specific analysis indicated that dogs with suspected serogroup Pomona infection were more likely to suffer from vomiting (P = .01), thrombocytopenia (P = .009), severe azotemia (P = .04), and hyperphosphatemia (P = .006) than dogs with other serogroups and were less likely to be discharged alive from the hospital (P = .03). This study suggests that only minor clinically relevant differences exist among serogroups. Leptospira serogroup Pomona caused more severe renal disease and was associated with a worse outcome compared with disease caused by other serogroups.