IgA nephropathy associated with thin basement membrane disease is reported in a 9-year-old female. The diagnosis of IgA nephropathy was made by means of an immunofluorescence investigation, which showed generalized diffuse mesangial deposits. Thin basement membrane disease was identified by electron-microscopic investigations, which disclosed thinning of the basement membrane of several capillary loops and prominence of the lamina densa. Her father, elder sister and younger sister were also found to have hematuria and her sisters were diagnosed as having thin basement membrane disease by renal biopsy. Patients with IgA nephropathy have focal thinning of the glomerular basement membrane, but we consider that urinalysis of the family needs to be done for the diagnosis of familial thin basement membrane disease, when diffuse thinning of the glomerular basement membrane is detected in such patients.