Anemia and neutropenia are commonly observed hematologic changes in patients with copper (Cu) deficiency, but thrombocytopenia is rarely found. A-69-year-old patient with postoperative small-bowel obstruction underwent laparotomy three times. Because of persistent obstruction, nasoduodenal suction was continued and total parenteral nutrition was instituted. Fifteen months after the initiation of total parenteral nutrition, the patient gradually developed pancytopenia (red blood cell count 222 x 10(4)/mm3, neutrophil count 1254/mm3, and platelet count 9.2 x 10(4)/mm3). The serum Cu level was 10 micrograms/dL and the serum ceruloplasmin level was less than 5 mg/dL. After 2 weeks of Cu supplementation in a daily dose of 20 mumol, the serum Cu level increased to 81 micrograms/dL and the serum ceruloplasmin level to 20 mg/dL. Hematologic values showed a dramatic response: red blood cell count increased to 362 x 10(4)/mm3, neutrophil count to 4819/mm3, and platelet count to 22.1 x 10(4)/mm3. The improvement of pancytopenia could be attributed to Cu supplementation. This is the first case report of Cu deficiency with pancytopenia during total parenteral nutrition.