We determined the clinical and immunopathological effects of dilazep hydrochloride (dilazep) on IgA nephropathy of ddY mice. Group I (early-treatment group, n = 10) was orally treated with 300 mg/kg body weight of this drug from 12 weeks of age until 60 weeks of age, and group II (late-treatment group, n = 10) was also treated with the same dosage of this drug from 20 weeks of age until 60 weeks of age. Group III (control group, n = 10) received drinking water. On immunofluorescence, distribution and intensity of IgA and C3 depositions in glomeruli of group I and group II animals were significantly decreased as compared with those in group III. The expression of fibronectin, laminin, or type IV collagen in glomeruli was basically similar in the three groups treated with or without dilazep. On light microscopy, the expansion of glomerular mesangial areas and the average number of intraglomerular cells were markedly decreased as compared with those in group III. The levels of urinary protein excretion in groups I and II were significantly lower than those in group III (p < 0.01 and p < 0.05). These findings suggest that treatment with dilazep might improve the clinical and immunopathological findings in IgA nephropathy of ddY mice.