Abstract: The clinical course of canine leptospirosis depends on the serotype of the causative agent and pathogenicity of the particular strain. In an effort to get accurate information about the leptospirae infestation of dogs and the part played by different serovars, 314 dogs including house pets, guard dogs and other working dogs from Kosice and other districts of Eastern Slovakia were examined clinically and serologically. For serological investigation the MAR test and 15 leptospira antigens after the method of Kmety and Bakoss (1978) were used. The positive reactions were confirmed by the presence of four serovars, the largest percentage being L. icyterohaemorrhagiae in 46.1 percent and L. grippotyphosa in 35.9 percent. The serovars L. bratislava in 10.3 percent and L.canicola in 7.7 percent were found from all the positive dogs. We could hypothesize that 3 case of positive serovar L. canicola could be the result of a modification of the epidemiological situation of canine leptospirosis or due to post-vaccination responses. The casuistic case of acute leptospirosis here described referred to a hepato-renal syndrome and cardiac insufficiency.