Patients undergoing continuous ambulatory peritoneal dialysis (CAPD) who developed sclerosing encapsulating peritonitis (SEP) were retrospectively studied in 130 centers in Japan. Among 6,923 patients undergoing CAPD between 1980 and 1994 only 62 (0.9%) given CAPD developed SEP. There were 38 men and 24 women, ranging in age from 20 to 87 years (average age, 48.3 years). These 62 patients developed SEP 10 to 138 months (average, 65.4 months) after starting CAPD. The average frequency of peritonitis before developing SEP was 3.3 times. Five of the 62 patients with SEP had no history of peritonitis, and 27 (43.5%) of them died of various causes in the study period. The major causes of death were almost invariably related to problems concerning bowel obstruction or complications of surgery, such as malnutrition or septicemia. It was concluded that SEP is one of the most serious complications of CAPD, and constant surveillance is necessary to detect SEP in patients during CAPD.