We hypothesized that intraperitoneal air might be one of the causes of peritoneal fluid eosinophilia. To test our hypothesis, we injected 100–500 ml of sterile air intraperitoneally into 5 patients receiving continuous ambulatory peritoneal dialysis (CAPD). All patients responded with a transient increase in peritoneal fluid nonerythrocyte cell count (peak counts ranging from 23 to 335 cells/mm3, mean peak count 140 ± 125) lasting 4 days (after injection of 100 ml of air) to 7 weeks (after injection of 500 ml of air). In 2 patients, the cells were predominantly monocytes(80 ± 6.5%), whereas in 3 patients, eosinophils predominated (63 ± 12%), while monocytes (30 ± 19%) also increased. Resolution of peritoneal fluid pleocytosis correlated temporally with absoption of subdiaphragmatic air. Our results suggest that intraperitoneal introduction of air into CAPD patients can induce peritoneal fluid eosinophilia and/or monocy