Astroviruses are a well-known cause of gastroenteritis in humans and many domestic animals. More recently, they have emerged as a cause of encephalitis in cattle and other species. Encephalitis is an economically important disease in cattle due to the death of animals. There is a zoonotic concern as many causes of encephalitis in cattle can cause disease in humans. It is therefore essential to determine the causes of encephalitis and their relative importance in a population. This study sought to determine the frequency of bovine astrovirus infection in cases of encephalitis in adult cattle in Ontario and to examine the pathologic findings in positive cases. Thirty-five cases of idiopathic lymphocytic encephalitis were retrieved from the Animal Health Laboratory / Ontario Veterinary College archives. As controls, 32 animals with non-lymphocytic encephalitis and 40 animals with no neurologic disease or encephalitic lesions were selected. All animals were screened using RT-qPCR for bovine astrovirus. No controls tested positive for bovine astrovirus. Four of the 35 animals (11%) with lymphocytic encephalitis tested positive for astrovirus. The four positive cases of astrovirus encephalitis were investigated with immunohistochemistry. The perivascular cuffs and glial nodules were composed overwhelmingly of CD3+ T lymphocytes. To investigate the historic role of bovine astrovirus encephalitis in Ontario cattle, an additional 21 cases from an older archive (1988-1998) were tested for bovine astrovirus, with 7 cases testing positive. In total, 56 cases of lymphocytic encephalitis were tested for astrovirus, and 11 tested positive (19.6 %). For the remaining cases of idiopathic lymphocytic encephalitis, a PCR panel was performed to search for other known causative agents. This included rabies virus, WNV, EEEV, OHV-2, BVDV, BHV-1, Listeria monocytogenes, and Chlamydia species. Nine of these 45 remaining cases (20 %) tested positive for other agents (EEEV, L. monocytogenes, BVDV, BHV-1, and OHV-2), leaving 36 of the initial 56 idiopathic cases of encephalitis (64 %) without a definitive diagnosis. Overall, 19 % of cases of lymphocytic encephalitis without a previously identified cause in Ontario cattle, from 1990-2019, tested positive for bovine astrovirus. This virus is therefore an important causative agent of fatal lymphocytic encephalitis in Ontario.