Peritonitis due to viscus perforation in peritoneal dialysis (PD) patients can be catastrophic. We describe the first reported case of perforated peptic ulcer (PPU) in a PD patient. This 78-year-old man presented with a 1-day history of mild abdominal pain. He had been receiving nocturnal intermittent PD for 2 years and had ischemic heart disease and cirrhosis of the liver. Pneumoperitoneum and peritonitis were documented, but the symptoms were mild. The "board-like abdomen" sign was not noted. Air inflation and contrast radiography indicated a perforation in the upper gastrointestinal tract, and laparotomy disclosed a perforation in the prepyloric great curvature. Unfortunately, the patient died during surgery. This case illustrates that the "board-like abdomen" sign may be absent in PD patients with PPU because of dilution of gastric acid by the dialysate. Free air in the abdomen, although suggestive of PPU, is also not uncommon in PD patients without viscus perforation. Because PD has to be discontinued after laparotomy and exploratory laparotomy may be fatal in high-risk patients, other diagnostic methods should be used to confirm viscus perforation before surgery. PPU, which can be proved by air inflation and contrast radiography, should be suspected in PD patients with pneumoperitoneum and peritonitis.