In ten patients undergoing maintenance peritoneal dialysis, large numbers of eosinophils were found in the peritoneal fluid. A few of the affected patients complained of episodic abdominal pains, but there was no correlation between abdominal symptoms and the number of peritoneal fluid eosinophils. Microorganisms failed to grow on cultures of the peritoneal fluids, and results of tests for endotoxin were negative. The cause of eosinophilia could not be determined. Peritoneal fluid eosinophil counts were noted to be elevated soon after catheter insertion and initiation of peritoneal dialysis. In some patients, peritoneal fluid eosinophil counts spontaneously returned to normal despite continued peritoneal dialysis.