To determine whether immune system disorders are involved in the exacerbation of IgA nephropathy, the immunoglobulin production of peripheral blood mononuclear cells obtained from 45 IgA nephropathy patients was measured and then compared with that of healthy individuals. The level of IgA production was classified into an elevated group and a non-elevated group and comparisons were made with various clinical factors considered to be related to exacerbation of this disease. The results indicated that although there was no significant difference in immunoglobulin production of the peripheral mononuclear cells between IgA nephropathy cases and healthy individuals in the group not stimulated with pokeweed mitogen (PWM), the group stimulated with PWM revealed a production of IgA, IgG, and IgM which was significantly elevated (P less than 0.01). Also, within the group stimulated with PWM, hypertension, severe proteinuria and microscopic hematuria, elevated BUN and serum creatinine values, decreased 15-min PSP and creatinine clearance values, severe histological damage, and severe IgA deposition were observed more in the elevated IgA production group than in the non-elevated group. These findings suggest that an elevated IgA production plays an important role in the excerbation of this disease.