Symptoms due to renal lesions often disappear and smolder by successful treatment during the first episode of Wegener's granulomatosis (WG). It is indispensable to recognize an increase in disease activity and provide adequate treatment to inhibit the progression of renal dysfunction as well as regulate disease activity in patients with WG. We gave attention to red blood cells in urinary sediment (U-RBC), a criterion in the diagnosis of WG, to more exactly determine the degree of renal activity. Comparison of level of U-RBC with other laboratory data and renal histology was performed in the clinical course of six cases. The level of U-RBC altered reversibly during the period studied and correlated with the C-reactive protein level and erythrocyte sedimentation rate which reflect disease activity in cases with microhematuria before treatment. When U-RBC disappeared within eight weeks after treatment, glomerular histological change remained small. The reported cases revealed that the alteration in U-RBC was more sensitive and more specific for renal involvement than the other disease markers. In conclusion, the level of U-RBC reflects the degree of renal activity in WG. We propose that the quantitation of U-RBC is extremely useful in following patients with WG.