We describe clinical data on 9 patients with Behçet's disease with renal involvement (8 males, one female) and results of kidney biopsies in 4 patients. Renal involvement occurred in 9 of 120 patients (7.5%) followed at our center. Proteinuria was present in 8 patients (6 with quantitative 24 h urine protein with a range of 0.32-3.11 g/day, mean 1.7 g/day); hematuria occurred in one patient. Impaired renal function was encountered in 2 patients at presentation and in another patient at a later date. Major vascular involvement (major venous and/or arterial thrombosis) was present in 4 patients and sagittal sinus thrombosis in another patient. Measurement of complement C3, C4, and CH50 and immunologic tests for anticardiolipin, antineutrophil cytoplasmic, and antistreptococcal antibodies did not indicate a clear association with renal involvement. Kidney biopsies were performed in 4 patients. Mesangial proliferative changes were observed in 3 patients and evidence of immune complex deposition by immunofluorescence and electron microscopic examination was present in 2 patients. Amyloidosis was present in one patient who had vascular involvement and presented with nephrotic range proteinuria.