Follow-up studies on 3,195 patients from 161 centers in Japan undergoing continuous ambulatory peritoneal dialysis (CAPD) were performed for 1-104 months to clarify the incidence as well as the clinical features of acute hydrothorax. In these studies, 50 patients (1.6%) developed this complication. Twenty-seven (54%) were men, and 23 (46%) were women, ranging in age from 6 to 79 (average 49) years. The interval between onset of CAPD and hydrothorax ranged from 1 day to 8 years. Four had left-sided, and 2 had bilateral hydrothorax, but the majority (88%) were right-sided. Dyspnea was experienced by 37 of these 50 patients, but the remaining 13 (26%) patients were asymptomatic. Hydrothorax was fully resolved in 27 of them following a brief interruption of CAPD or the combined use of small exchange volumes in a semi-sitting position and pleurodesis with tetracycline or other agents. The remaining 23 patients (46%) were switched to hemodialysis permanently. Despite recurrence, 1 patient continued successfully on CAPD. It was concluded that acute hydrothorax is one important possible complication, although the risk may be low. Constant surveillance is necessary to detect pleural effusions in patients during CAPD.