Filarial glomerulonephritis was studied using Dirofilaria immitis infected dogs. Of 34 infected dogs examined, 15 dogs (44.1%) had histopathological lesions in the kidney. These lesions included an increased number of mesangial cells and increased thickness of the matrix, the infiltration of the small round and plasma cells into the interstitium and thickening of the basement membrane. Deposits of IgG were demonstrated in the infected dogs, whereas C3 deposits were found in all dogs. Combined immunoglobulin and complement deposits were not always found in the dogs with histopathological lesions. The mean concentration (expressed as absorbance) of circulating immune complexes (CIC) was 0.675 +/- 0.517 in infected dogs, and 0.132 +/- 0.092 in uninfected dogs. Although there was significant difference in the level of CIC between infected and uninfected dogs (P less than 0.001), 11 dogs (32.4%) in infected group were negative. Otherwise, the CIC levels were correlated to the adult worm burden (r = 0.848; P less than 0.001) but not to the number of circulating microfilariae (mf) (r = 0.398; P less than 0.05). Transfer of mf to 7 naive dogs was performed to clarify the role of mf in the pathogenesis of filariasis. Antibodies to crude mf antigen became detectable two weeks after the transfer. Neither pathologic findings nor deposits of IgG and C3 in the kidney were found in dogs examined 20 days or 70 days after transfer. There was no evidence that histopathological lesions were induced by live mf, suggesting that adult worm burdens may be more closely related to filarial nephropathy.