T cell subsets, serum immunoglobulin (IgM) level, IgM-bearing lymphocytes, and circulating immune complexes (CIC) were studied in 12 children who suffered from IgM mesangial nephropathy (IgMN) during the acute nephrotic phase, in remission and relapse. Frequent relapses were observed in 11 cases, and 1 was partially responsive to steroid treatment. IgMN was diagnosed by the consistent pattern of IgM deposition by all four FITC-labelled antihuman IgM antibodies from rabbits and goats supplied by four different companies and by the 100% positivity of electron-dense mesangial deposits in an identical localization and distribution pattern of kidney biopsy specimen. CIC were detected by the 3.5% polyethylene glycol method. In sera from 12 patients IgM CIC were detected in 8 cases during the acute nephrotic phase. High levels of C3 CIC and C4 CIC were also found in these cases during the acute nephrotic phase. The CIC were undetectable in remission. Only 3 cases were detectable at low levels of IgM CIC during the second relapse. High serum IgM levels and IgM-bearing lymphocytes were noted in these patients. The patients also had a significant increase of OKT8 cells and a decrease in the OKT4/OKT8 ratio during the acute phase and in relapse. Taken together, the immunopathologic and clinical features suggest that IgMN is a disease entity with a chiefly classical pathway activation of complement components. The correlation between the changes of T cell subsets and the disease activity in IgMN suggests that this may serve as a therapeutic and prognostic guide.