Steroid responsive meningitis-arteriitis (SRMA) is a well-known disease in dogs, but the aetiology and pathogenesis are not yet understood. In the peripheral blood an overrepresentation of B cells was found. In the present study we therefore evaluated the distribution of lymphocyte subsets in SRMA in paraffin-embedded tissue sections directly at the lesion sites and compared the results to different canine encephalitides. An intriguing finding was that the B cell/T cell distribution varied depending on the aetiology of the disease: in viral encephalitides, T cells were the predominant cell population in perivascular cuffs, whereas in protozoal and bacterial diseases B cells prevailed. In SRMA an overrepresentation of B cells occurred in meningeal lesions, as already found in the peripheral blood. The distribution of lymphocyte subsets was similar to bacterial and protozoal diseases and was not a unique phenomenon for this specific inflammatory lesion in the canine central nervous system (CNS). Multiple mechanisms seem to be responsible for recruitment and activation of different leukocyte subsets after alteration of the CNS tissue by an environmental factor. A specific finding in SRMA was that the distribution of T and B cells depended also on the lesion site. In contrast to meningeal lesions, in inflamed arteries T cells were the only lymphocyte population found. In these vessels, diffuse infiltration with immunoglobulins was revealed. Inactivated or resting lymphocytes and large granular lymphocytes occurred in each of the diseases examined. These similarities between SRMA and infectious CNS diseases of the dog support earlier suggestions that the disease is somehow triggered by a hitherto unknown environmental factor which leads to the dysregulation of the immune system.