A new type of hereditary glomerulopathy was observed in ten children presenting with early and progressive glomerular symptoms, often associated with hypertension. Light microscopy showed a diffuse increase in the mesangial matrix and generalized widening of the capillary walls. Electron-microscopic examination of renal tissue, after phosphotungstic acid treatment, revealed the presence of fibrillar collagen within the mesangial matrix and the subendothelial aspect of the glomerular basement membrane, adjacent to normal lamina densa. Immunohistochemical studies identified the fibrillar collagen not usually present within the glomerular extracellular matrix as type III collagen. Clinical and family studies ruled out the diagnosis of nail-patella syndrome, an autosomal dominant disorder with typical extrarenal symptoms, which is also characterized by the presence of fibrillar collagen within the glomerular basement membranes. The poor renal outcome, the possible extrarenal haematological and pulmonary involvement and the transmission as an autosomal recessive trait strongly suggest that collagen type III glomerulopathy is a new type of hereditary disease. From the high incidence of superimposed haemolytic uraemic syndrome in patients or their siblings, it may be hypothesized that collagen type III glomerulopathy is the underlying defect in some of the familial cases of haemolytic uraemic syndromes.