A prospective study on peritoneal eosinophilia was conducted in 23 continuous ambulatory peritoneal dialysis (CAPD) patients for a mean period of 7.9 months. Peritoneal eosinophilia as defined by peritoneal eosinophil count exceeding 100/mm3 was found in 60.8% of patients. Most developed peritoneal eosinophilia within 3 months of the initiation of dialysis, although the phenomenon could occur as early as one day or as late as 6 months after dialysis. Fifty-seven percent of those with peritoneal eosinophilia also had peripheral blood eosinophilia. Although most peritoneal eosinophilic episodes subsided in a month, in one patient the process grumbled on for 150 days. The number of peritonitis episodes was not significantly different between patients with peritoneal eosinophilia and those without. The only distinction between the two groups of patients was that those who developed peritoneal eosinophilia had a significantly (P = .002) higher serum IgE concentration initially as well as throughout the period of observation.