5-Fluorouracil (5-FU) remains the most effective chemotherapeutic agent in the management of patients with metastatic colorectal cancer. Leucovorin enhances its efficacy, but also its toxicity. Cited data suggest modulation of 5-FU toxicity by high dose allopurinol. In a prospective randomized trial we assessed the ability of allopurinol in a conventional dose to modulate the toxicity of 5-FU-leucovorin combination without compromising its efficacy in 50 patients with advanced colorectal cancer. Twenty-seven patients were randomized for allopurinol but had no benefit in terms of response or reduced toxicity over the other 23. Survival of responders with colon cancer was longer than that of non-responders (p = 0.013). Although allopurinol failed to reduce 5-FU-leucovorin toxicity, it did not lower its expected efficacy.