During a mean therapeutic follow-up of 4 years, 25 out of 61 patients with Wegener's granulomatosis had one or several relapses. As in 7 of them the initial diagnosis had proved erroneous, this study concerns the remaining 18 patients who together totalled 31 relapses after the correct diagnosis was made. Twenty relapses occurred early on, within 4 months of a change in treatment, and they corresponded to recurrent activity of the disease. Ten relapses occurred later on, after treatment had been discontinued or kept at maintenance dose level. The relapses appeared as early as the first year in 41 percent of the cases. The ENT region was affected in 61 percent of the patients. Nine relapses occurred in the absence of treatment, including 6 which took place 1 to 8 years after treatment. Three relapses occurred less than 3 months after a prednisone and oral cyclophosphamide therapy was initiated. Compared with the group of patients without relapse, those in the relapse group were younger, more often treated with azathioprine and cyclophosphamide administered intravenously, and followed up for a longer period; their probability of survival was also higher. In case of early relapse after withdrawal of treatment the mean duration of that treatment had been shorter than in the absence of relapse. None of the patients whose treatment had been stopped more than 20 months after its onset had an early relapse. These data prompted us to make the following proposals: initially, cyclophosphamide should be administered as bolus injections (the time required to obtain results is then shorter than with the oral route), and maintenance therapy should last at least 20 months. Azathioprine should be used only when cyclophosphamide is contra-indicated. The relative indications for intravenous and oral cyclophosphamide remain to be determined. Discontinuing all treatments is a difficult decision to make, but even then the patients should be under close supervision in view of the possibility of late relapses.