Canine leptospirosis has been known as Stuttgart disease since 1898, and dogs are considered to be the second principal source of infection in man. The isolation of a pathogenic serovar from dog urine that was diagnosed clinically and laboratorial as having leptospirosis and its utilization to test serological samples of human and canine cases of leptospirosis, has demonstrated its importance to the ecosystem of the southern region of Brazil. The results of the serological microscopic agglutination test indicated that 100% of human serum samples from 12 patients from the serum bank of 2001 at the Center for Control of Zoonoses, that had titers between 25 and 3,200 with the canicola serovar, and 72% of 105 canine serum samples from the same serum bank, also reacted with the new isolate. The mean and median titers of the human serum samples tested with the battery of antigens recommended by WHO was 630 and 100 respectively, and when tested with the isolate it was 1,823 and 400. In the dog sera, the values were respectively 347 and 100 with the battery, and 1,088 and 200 with the isolate.