The findings are reported in 38 patients with familial hematuria. In 10 of the 24 families investigated, a familial incidence of hematuria was revealed only by routine urinalysis in first-degree relatives. Where there was either neurosensory deafness of heavy proteinuria in the patient or family, or a history of chronic renal failure in the family, the patient generally ran a progressive clinical course. Light microscopy (LM) of renal biopsy specimens revealed little abnormality in young children, but segmental glomerular sclerosis was frequently observed in older patients. The most characteristic change, observed on electron microscopy (EM) in 27 out of 31 renal biopsies was complex replication of the lamina densa of the capillary basement membrane, to form a "basket weave" pattern. Families with and without deafness (groups 1 and 2) were both considered to fall within the spectrum of Alport's syndrome, although the presence of deafness adversely affected the prognosis. In contrast, patients from families showing neither deafness, heavy proteinuria nor chronic renal failure (group 3) ran a non-progressive course. Their biopsies showed little or no glomerular changes other than attenuation of the lamina densa on EM. In Alport's syndrome, deafness, heavy proteinuria, segmental sclerosis and foam cells were not often present before the age of 10 years, whereas the "basket weave" alteration of the lamina densa was found in all three children biopsied under 5 years of age. We therefore emphasize the importance of EM in the differential diagnosis from benign familial hematuria.