ABSTRACT.We have investigated 65 children with isolated hematuria persisting for at least a year. Renal biopsy specimens were studied by light microscopy, electron microscopy and immunofluorescence with antisera specific against basement membrane components. The majority of the biopsies (62/65) showed variable histologic abnormalities. Four categories could be distinguished on combined histological and clinical criteria: Alport syndrome (n=8), benign hematuria (n=33, familial in 23), IgA nephropathy (n = 16) and increase in mesangial cells and matrix (n=5). On the basis of our results, we suggest that a renal biopsy can establish diagnosis and prognosis in those children with isolated hematuria where the family history is negative. If the family has adult male individuals with isolated hematuria, a biopsy can usually be avoided, since this family history effectively excludes Alport syndrome. The use of antisera against basement membrane components did not allow a differentiation between Alport syndrome and benign hematuria. Goodpasture serum immunofluorescence was variable in the former and normally present in the latter.