The therapeutic activity of 5-FU in large bowel cancer is enhanced by increasing the intracellular pool of reduced folates. We treated 45 patients with advanced colon cancer with HDFA and 5-FU for 5 consecutive days. None had been given previous radio- or chemotherapy. All had measurable disease. Not one complete response was observed. Thirteen of the 39 evaluable patients showed partial response. Median duration of response was 9+ months. The probability of 50% survival was 15 months for all evaluable patients. There was no case of severe toxicity and the principal toxic effects were oral mucositis and diarrhea. To date, HDFA + 5-FU is one of the most effective treatments for large bowel cancer.