PURPOSE: In randomized trials, patients with stage III colon cancer who received 6 months of fluorouracil (FU)-based adjuvant chemotherapy had better survival than patients who did not. However, little is known about the predictors of, or the survival associated with, duration of chemotherapy in the community. PATIENTS AND METHODS: The linked Surveillance, Epidemiology, and End Results-Medicare database was used to identify individuals > or = 65 years of age diagnosed with stage III colon cancer between 1995 and 1999. We used logistic and Cox proportional hazards regression models to analyze factors associated with early discontinuation of FU-based chemotherapy among these elderly colon cancer patients. RESULTS: Among 1,722 patients who received 1 to 7 months of FU-based chemotherapy, older age, being unmarried, and having comorbid conditions were associated with receiving less than 5 months of treatment. Among the 1,579 patients who survived > or = 8 months, the 1,091 (69.1%) who received 5 to 7 months of treatment had lower overall (hazard ratio [HR], 0.59; 95%, CI 0.49 to 0.71) and colon cancer-specific (HR, 0.53; 95% CI, 0.43 to 0.66) mortality than the 488 (30.9%) who received 1 to 4 months of treatment. CONCLUSION: More than 30% of elderly patients who initiated FU-based chemotherapy for stage III colon cancer and survived for at least 8 months discontinued treatment early. Mortality rates among such patients were nearly twice as high as among patients who completed 5 to 7 months of treatment. If the association we observed between duration of treatment and survival is confirmed, additional investigation is warranted to determine whether dose-intensity, cumulative dose, or other factors related to receipt of full adjuvant treatment are responsible.