The clinicopathological picture of 'isolated C3 mesangial nephritis' was studied in our case records. Focal and segmental or generalised deposits of C3 in the mesangium were found in 12 of 157 (7.6%) patients with primary glomerulonephritis. The clinical picture, similar to Berger's disease, was characterised by episodes of gross haematuria and/or persistent or recurrent microhaematuria and/or proteinuria. Arterial hypertension and mild renal failure were observed in one case. Light-microscopy showed minor glomerular changes such as focal and segmental increase of mesangial matrix and mesangial hyperplasia. During the short-term follow-up (median 25.5 months) no deterioration of renal function was observed. The clinical course and short-term prognosis suggest that this form of glomerulonephritis is benign.