Twenty-three cases of IgM associated primary diffuse mesangial proliferative glomerulonephritis are presented. In 18, IgM was the sole localising host immunoglobulin, and it was the predominant globulin in five; C3 was also present in 18. Light microscopy revealed variable diffuse and global mesangial proliferation in all cases, with additional focal global sclerosis in 16, focal segmental sclerosis in 15, and small capsular crescents in seven. Material for electron microscopy was available from 19 patients; in 13, occasional intramesangial electron dense deposits were identified, and in 18 there were irregular, rather ill defined areas of increased electron density in mesangial regions. Clinically, 14 patients presented with the nephrotic syndrome, and nine had asymptomatic proteinuria. During follow-up, only 10 patients showed no change in renal function or improved; the remainder showed increasing hypertension and/or renal function deterioration and four developed end stage renal failure. It is suggested that IgM associated mesangial proliferative glomerulonephritis should be considered as a distinct clinicoimmunopathological entity.