BACKGROUND: Since 1990, the recommended adjuvant therapy for patients with surgically resected node-positive colon cancer has been 5-fluorouracil (5-FU), usually in combination with leucovorin or levamisole. The purpose of this study is to assess the distribution of adjuvant 5-FU treatment in the elderly. METHODS: The Surveillance, Epidemiology and End Results-Medicare database provides population-based information on cancer patients, representing approximately 14% of the United States population, along with health care utilization data from Medicare claims files. We studied patients with node-positive colon cancer diagnosed between 1992 and 1996 who survived at least 120 days beyond diagnosis (N = 4998). RESULTS: About 50% of elderly patients received 5-FU within 4 months of diagnosis. The proportion of patients treated with 5-FU increased by about 10% from 1992 to 1996. In a multiple logistic regression model, 5-FU treatment was less likely to be given to older patients (compared with those aged 65-69 years, the odds ratio (OR) [95% CI] was 0.82 [0.67-1.00] for ages 70 to 74 years, 0.47 [0.39-0.57] for ages 75 to 79, 0.17 [0.13-0.20] for ages 80 to 84, and 0.04 [0.03-0.05] for ages 85 to 88 years. Non-Hispanic black patients were less likely to be treated than non-Hispanic white patients (OR 0.46 [0.36-0.59]); patients with more than three positive lymph nodes were more likely to be treated than those with three or less, and those with comorbid conditions were less likely to be treated than those without such conditions. CONCLUSIONS: Despite its proven efficacy in reducing colon cancer mortality, 5-FU-based chemotherapy is not widely used among apparently eligible patients over age 65. Efforts are needed to ensure that elderly and non-Hispanic black patients receive appropriate treatment.