Identical 7-year-old twin boys each had a proved adenovirus infection at the same time. A few days later one developed florid Henoch-Schönlein purpura, severe alimentary tract symptoms, and transient joint symptoms. He had an acute nephritic syndrome, which progressed to nephrotic syndrome and renal insufficiency. Biopsy showed severe proliferative glomerulonephritis with crescents and marked deposition of IgA, IgG, C3, and fibrin. The second twin had hematuria and abdominal pain but no rash. Biopsy showed mesangial proliferative glomerulonephritis with mesangial deposits of IgA and, to a lesser extent, IgG and C3. The appearance was characteristic of Berger disease, and the subsequent clinical course has been that of symptomless microscopic hematuria and recurrent macroscopic hematuria with normal renal function. Immunologic studies have not revealed why these identical twins responded differently to the same provocation. Perhaps Berger disease may be considered a variant of Henoch-Schönlein nephritis.