Diagnosis of Neospora caninum associated bovine abortion is often made by histological examination of fetal tissues, although this procedure is not very sensitive. One hundred and forty-four undiagnosed bovine abortion cases were evaluated for N. caninum associated bovine abortion using a revised diagnostic protocol. As a result, 12 (8.3%) of these previously undiagnosed bovine abortion cases were definitively diagnosed as N. caninum positive. The 12 new positive cases included 7 that had exhibited histological lesions, and 5 that had not exhibited histological lesions when examined prior to this study. None of the 12 cases had been immunohistochemically (IHC) stained prior to this study. Also, IHC staining implemented during this research revealed tachyzoites without associated histological lesions in at least 1 tissue from 69 (47.9%) of 144 aborted bovine fetuses, and positive IHC staining of cytoplasmic N. caninum antigen in macrophages in at least 1 tissue from 44 (30.6%) of 144 aborted bovine fetuses. These results demonstrate the necessity for more aggressive evaluation of bovine fetuses for neosporosis.