This study was designed to determine the seroprevalence of brucellosis and leptospirosis in aborted dairy cattle. For this purpose, serum samples were collected from 163 aborted dairy cattle that had no history of vaccination against either brucellosis or leptospirosis. The antibodies against B. abortus were detected in these serum samples as 68.1 %, 65.6 %, 58.9 % and 55.2 %, respectively by the Competitive Enzyme-Linked Immunosorbent Assay (C-ELISA), Complement Fixation Test (CFT), Rosebengal Plate Test (RBPT) and Serum Agglutination Test (SAT). In the study, the serum samples were also collected from 15 healthy dairy cattle which had been vaccinated with reduced dose of B. abortus S-19 vaccine six months before the collection of the serum samples. These serum samples were used to compare the differences in the antibody titers between the vaccinated and naturally infected dairy cows by using C-ELISA and CFT. It was found that the animals vaccinated with this vaccine should be revaccinated before six months.