Objective: To estimate the extent to which abortion in dairy cows was associated with of Neospora caninum (N. caninum) and to determine the risk factors of neosporosis in dairy farms from 9 provinces in Iran. Methods: Polymerase chain reaction (PCR) test was used to detect Neospora infection in the brain of 395 bovine aborted fetuses from 9 provinces of Iran. In addition, the brains of aborted fetuses were taken for histopathological examination. To identify the risk factors associated with neosporosis, data analysis was performed by SAS. Results: N. caninum was detected in 179 (45%) out of 395 fetal brain samples of bovine aborted fetuses using PCR. Among the PCR-positive brain samples, only 56 samples were suited for histopathological examination. The characteristic lesions of Neospora infection including non-suppurative encephalitis were found in 16 (28%) of PCR-positive samples. The risk factors including season, parity of dam, history of bovine virus diarrhea and infectious bovine rhinotracheitis infection in herd, cow's milk production, herd size and fetal appearance did not show association with the infection. This study showed that Neospora caused abortion was significantly more in the second trimester of pregnancy than other periods. In addition, a significant association was observed between Neospora infection and stillbirth. Conclusions: The results showed N. caninum infection was detected in high percentage of aborted fetuses. In addition, at least one fourth of abortions caused by Neospora infection. These results indicate increasing number of abortions associated with the protozoa more than reported before in Iran.