A 51-year-old woman had been suffering from blood-stained purulent sinusitis and antibiotic-resistant bouts of fever for 4 months. She had microhematuria and serological evidence of inflammation (erythrocyte sedimentation rate [ESR] 92/135 mm, C-reactive protein 5.0 mg/dl). When she was admitted to hospital suspected of having postinfectious glomerulonephritis she complained of spontaneous colic-like pains in the left flank. Within one day the haemoglobin concentration fell from 10 to 6.5 g/dl. Ultrasound and computed tomography demonstrated a large space-occupying lesion around the left kidney. At operation this was found to be a rupture of the kidney with perirenal bleeding which was treated without removing the kidney. No biopsy was taken, but serological tests showed antineutrophil cytoplasmatic antibodies (cANCA), indicating Wegener's granulomatosis as the cause of the compensated renal insufficiency and spontaneous renal rupture. Under immunosuppressive treatment the inflammatory signs (ESR 18/44 mm), fever, chronic maxillary sinusitis, raised serum creatinine concentration and the ANCA titre all regressed, while proteinuria of about 4 g/24 h persisted. There was no recurrence during a follow-up period of 15 months. Serological signs of marked inflammatory activity, urinary sediments of nephritis and spontaneous retroperitoneal bleeding should suggest that, in addition to lupus erythematodes and panarteritis nodosa, Wegener's granulomatosis be included in the differential diagnosis.