We have described a case of massive unilateral hydrothorax during chronic peritoneal dialysis. The communication between the peritoneal and right pleural cavities was confirmed by injection into the peritoneal cavity of methylene blue, which promptly appeared in the pleural cavities. Peritoneal dialysis was continued, using decreased dialysate and a semisitting position. The possibility of continuing peritoneal dialysis in the patient with acute hydrothorax and in whom hemodialysis may be contraindicated is demonstrated.