Randomized trials conducted in the 1980s have established the effectiveness of 5-fluorouracil-based adjuvant chemotherapy in treating stage-III colon cancer. However, the initiation of adjuvant chemotherapy is just the first step for survival improvement. Little is known about the actual completion rate of such a therapy in the community. The objectives of this study were to measure the initiation and completion rate of adjuvant chemotherapy and to identify the associated factors. We studied 12,265 patients aged 65+ diagnosed with stage-III colon cancer between 1991 and 2005 who were identified from the Surveillance, Epidemiology, and End Results-Medicare linked database. Chemotherapy initiation was defined as at least one claim indicating the use of chemotherapy. The first and last claims were used to measure the length of chemotherapy. A complete course of chemotherapy was defined as 8-13 months for 1991-1995 cohort and 5-7 months for 1996-2005 cohort according to clinical guideline. Of the 12,265 patients, 64.4% received adjuvant chemotherapy within 3 months after tumor resection. Among those who had chemotherapy initiated, 62.2% (or 38.0% of 12,265 patients) received a complete course of chemotherapy. Patient's age at diagnosis, marital status, and comorbidity score were the significant predictors for chemotherapy initiation. These variables remained significant in predicting chemotherapy completion after adjusting for year of diagnosis and other factors. In conclusion, initiation and completion of chemotherapy was largely influenced by patient's age, marital status and comorbidity. Further investigation is needed to explore the cause of these differences in adherence to standard treatment that is essential for better quality of cancer care.