Mesangioproliferative glomerulonephritis of varying severity was found by renal biopsy in 86.2% among 130 patients with asymptomatic haematuria. 3.2% had benign nephrosclerosis, 3.2% had benign nephrosclerosis, 1.6% had interstitial nephritis and one patient showed a previously undiagnosed perimembranous glomerulonephritis. Normal renal parenchyma was observed in only 6.9% of cases. Iummunohistological findings were positive in 25.7% of investigated cases. The majority were IgA deposits combined with IgG and C3 complement. The intensity of haematuria is not correlated with the type and severity of histological changes. "Physiological" haematuria (erythrocytes less than 7/ml) which is usually considered normal must be re-evaluated as a consequence of these histological findings. After exclusion of urological or extrarenal disease only histological investigation of the kidneys will bring the final diagnosis. Repeated radiographical or urological investigations can thus be avoided. The risks of percutaneous renal biopsy under fluoroscopic or sonographic control are decidedly less than the information gained from histological evaluation. The different histological findings indicate that isolated haematuria should be considered only as a symptom and not as a uniform disease indicating focal nephritis.