The course of idiopathic membranoproliferative glomerulonephritis (MPGN) in 41 Japanese children (21 boys, 20 girls) is reported. The mean follow-up period was 8 years, 9 months; 29 children with MPGN (71%) were identified by school urinary screening; 32 patients had type I MPGN, 2 type II and 7 type III; 10 patients were treated with multiple low-dose cocktail therapy (MLD), 8 with MLD followed by high-dose alternate-day (ALD) prednisolone and 21 with high-dose ALD prednisolone alone. In 1 patient, MPGN progressed to end-stage renal failure. The serum creatinine level in all of the remaining 40 patients was < or = 1.3 mg/dl at the last follow-up. Urinalysis was normal in 24 (59%). Of the 17 patients who still had urinary abnormalities, 4 had nephrotic syndrome. The incidence of remission of urinary abnormalities was highest in the patients treated with high-dose ALD prednisolone. Rebiopsy was performed in 33 patients, and revealed slight histological improvement in 11 (33%) patients, moderate improvement in 8 (24%), marked improvement in 5 (15%) and deterioration or no improvement in 9 patients (27%). Relatively few side effects of treatment were observed. The superior outcome of the MPGN patients in this compared with other studies may be the result of earlier detection and treatment.