On hundred and fifteen renal biopsies performed in 112 patients with mesangial IgA nephropathy were reviewed and the histological disease patterns correlated with the clinical features at the time of initial biopsy. To determine the significance of macroscopic haematuria in this disease, specific comparisons were made between patients with a history of episodes of macroscopic haematuria and those with only microscopic haematuria. The mean age at initial biopsy was 38.3 years (90 males, mean 40.3 years; 22 females, mean 30.2 years). Histological examination showed 9 patients (8%) with class I disease (mesangial matrix expansion alone); 43 patients (38%) with class II disease (diffuse mesangial proliferation); 60 patients (54%) with class III disease (focal and segmental proliferation), including subsets of 20 patients (16%) with segmental sclerosis and/or synechiae and 23 patients (21%) with crescent formation. Class III disease and crescent formation correlated with an increased frequency of capillary loop IgA and glomerular fibrin deposition and with the presence of subendothelial and subepithelial deposits. The degree of renal impairment and the incidence of hypertension were increased in class III disease. Macroscopic haematuria patients were younger (mean 31.1 vs. 43.0 years; p less than 0.001), had less severe renal impairment (mean creatinine 116.2 vs. 213.3 mumol/l; p less than 0.001) and less class III disease (48 vs. 58%; p less than 0.05). The incidence of crescentic disease was equal in macroscopic (17%) and microscopic (23%) haematuria. Eventual progression to end-stage renal failure occurred in 12 patients (11%) and correlated with crescentic disease, renal impairment, hypertension and heavy proteinuria at the time of diagnosis.(ABSTRACT TRUNCATED AT 250 WORDS)