BACKGROUND The use of permanent intravenous access devices for chemotherapy administration has become a common practice in clinical oncology. Therefore, awareness of potential complications is important. The authors previously reported the use of high dose 5-fluorouracil (5-FU) (2600 mg/m2) and leucovorin (500 mg/m2) as a weekly 24-hour infusion for patients with colorectal carcinoma. In this report, a new complication of permanent indwelling catheters with high dose 5-fluorouracil (2600 mg/m2) and leucovorin (500 mg/m2) as a weekly 24-hour infusion for colorectal carcinoma is described. METHODS Twenty-two patients in the previous Phase II trial on weekly high dose 5-FU and leucovorin were included in this study. All patients had either a single-lumen Port-o-cath (Pharmacia Deltec, St. Paul, MN) or Hickman catheter (Travenol Laboratories, Deerfield, IL). Occluded catheters were explanted, and the material found in their lumen was analyzed using infrared spectroscopy. RESULTS Eleven of 22 patients had catheter blockage, and calcium carbonate formation (Calcite 100%) was identified within these catheters. CONCLUSIONS Calcite formation causing catheter occlusion is a new and important complication resulting from using intravenous access devices for chemotherapy administration. Oncologists should be alerted to this phenomenon when high dose 5-FU and leucovorin are administered for 24 hours by continuous infusion using a single-port port-o-cath.