Leptospirosis is a common disease in dogs, despite their current vaccination. Vet surgeons may use a serological test to verify their clinical observations. The gold standard is the Microscopic Agglutination Test (MAT). After infection, the dog produces agglutinating antibodies against the lipopolyosidic antigens shared by the infectious strain but also, after vaccination, against the lipopolyosidic antigens shared by the serovars used in the bacterins (Leptospira species serovars Icterohaemorrhagiae and Canicola in most countries). MATs were performed in a group of 102 healthy field dogs and a group of 6 Canicola-challenged dogs. A diagnosis algorithm was constructed based on age, previous vaccinations, kinetics of the agglutinating antibodies after infection or vaccination and the delay after onset of the disease. This algorithm was applied to 169 well-documented sera (clinical and vaccine data) from 272 sick dogs with suspected leptospirosis. Totally, 102 dogs were vaccinated according to the usual vaccination scheme and 30 were not vaccinated. Leptospirosis was confirmed by MAT in 37/102 (36.2 per cent) vaccinated dogs and remained probable in 14 others (13.7 per cent), thus indicating the permanent exposure of dogs and the weakness of the protection offered by the current vaccines to pathogenic Leptospira.