Leptospirosis is a zoonosis of worldwide distribution that has the dog as an important host, showing the importance of diagnosing and treating this animal that has a straight relationship with humans. A dog was attended at the Veterinary Hospital of the Federal University of Piauí in Teresina, Piauí, was present with history of hematuria, hyporexia and lethargy. Physical examination revealed lethargy, oral, ocular and penile icteric mucosa, pain at abdominal palpation and pyrexia. Hematologic analysis revealed hypochromic macrocytic anemia, thrombocytopenia, leukocytosis due to neutrophilia, and an increase in urea, AST, ALT, alkaline phosphatase and bilirubin values. The microscopic serum agglutination test showed a reactive result of Leptospira interrogans serovar Australis at the 1: 100 titre. The animal was effectively treated with Ampicillin, Doxycycline and supportive therapy.