12 patients with mesangial C3-glomerulonephritis, 7 women and 5 men, aged 16-42, mean age 29.7 years, were followed up for 1-18 years, mean time 5.7 years. The disease was manifested most often by macroscopic hematuria or low degree proteinuria. Single patients had arterial hypertension. In only one patient there was a nephrotic syndrome. For the period of observation, impairment of the renal function was found in none of the patients.