tNeosporosis caused by caused by the apicomplexan parasite Neospora caninum is one ofthe major causes of infectious abortion in bovines worldwide. A long-term prospectivestudy was performed in a dairy herd endemic for N. caninum in order to analyze the impactof neosporosis on the proportion of aborting cows. A total of 1078 pregnant cows weretested for presence of antibodies and the proportion of abortions was calculated. The over-all seroprevalence of N. caninum found in the herd was 35.5%. The percentage of abortionsin seropositive cows was 3 times higher than in their seronegative counterparts (21.6 and7.3%, respectively). No statistically significant association was found between the antibodylevel of positive during pregnancy and the proportion of aborting cows. However, 41.2%of the dams with antibody titers of 1:12,800 aborted. The risk of abortion for such damswas 2.7 times higher than for other seropositive cows which had lower titers of antibodies(p = 0.0072). In the follow-up examinations of the seropositive cows during several pregnan-cies, the overall percent of abortions observed was significantly higher than in seronegativeindividuals (49.3 and 16.9%, respectively; p < 0.0001). Moreover, the proportion of repeti-tive abortion observed was 5 to 1 (17.4 and 3.5%) in seropositive and seronegative dams,respectively (p < 0.001). The rate of vertical transmission in positive dams was 61.0% andit appeared to be directly associated with antibody levels: the higher the titer in the damsduring pregnancy, the higher the percentage of sero-positivity in their calves. Increased proportion of abortions was observed in seropositive cows both in summer and winter in comparison with spring and autumn. It was found that in seropositive cows, an increased number of pregnancies, which was directly related to the age of the dam, has been associ-ated with an increased number of abortions.