A series of 150 samples of renal tissue obtained by needle biopsy from patients with haematuria were examined immunohistologically. In 15 cases mesangial deposits of IgA were demonstrated. In most cases they were young patients in whom no hypertension, oedema or laboratory findings indicating renal damage could be detected one year later during a follow-up examination. Electron microscopical examination was carried out in the renal tissue of 7 such cases, and showed dense mesangial deposits and a marked subendothelial rarefaction within the basement membrane. The light microscopical findings were variable. Three cases showed minimal lesions, 5 mesangial proliferative glomerulonephritis and 7 mesangial proliferative glomerulonephritis with focal segmental sclerotic alterations within the glomeruli.