We tested the clinical and immunoregulatory effects of peritoneal dialysis and hemodialysis on a patient with familial Mediterranean fever (FMF), amyloidosis, and chronic renal failure. His frequency of FMF attacks during maintenance hemodialysis (no attacks in 21 months) was significantly less than during conservative medical therapy (10 attacks in 14 months, P < 0.00002) or during intermittent peritoneal dialysis (3 attacks in 4 months, P < 0.004). His mean (±SE) percentage suppressor cell function was significantly (P < 0.001) higher during hemodialysis (53 ± 5) than during conservative medical therapy (4 ± 3) or during peritoneal dialysis (2 ± 7) and was similar to the healthy untreated volunteers (46 ± 3). This suggests that his suppressor cell deficiency may be associated with the pathogenesis of his disease. One possible mechanism by which hemodialysis ameliorates FMF may be the correction of a suppressor cell abnormality.