METHODS A 58-year-old man was treated for abdominal and inguinal recurrence of a centroblastic non-Hodgkin lymphoma with high doses of methotrexate and cytosine arabinoside. A chest radiogram, taken on the 18th day of chemotherapy to exclude pulmonary infiltration, revealed pneumoperitoneum. The patients's complaints were merely of abdominal fullness and persisting diarrhoea. His general state was hardly impaired, the abdomen soft on palpation with active peristalsis. METHODS There was aplastic anaemia (haemoglobin 9.9 g/dl, erythrocytes 3.4 x 10(6)/microliters, white cells 1000/microliters, platelets 20,000/microliters. Plain film of the abdomen and abdominal computed tomography confirmed pneumoperitoneum. Intraluminal air had caused wall dissection in the ascending and transverse colon (pneumatosis coli as cause of the pneumoperitoneum). METHODS There was complete absorption of the free abdominal air by the 29th day of chemotherapy under parenteral nutrition and metronidazole (3 x 500 mg daily).