OBJECTIVES: Coloretal cancer (CRC) is one of the most common and costly cancer in the United States. 70% of the patients are diagnosed post 65 years old. This study used a large cohort of patients from 2005-2008 100% Medicare Institutional Inpatient and Outpatient data to estimate the cost of different treatments and understand the chemotherapy use for CRC patients. METHODS: 203,532 (50.8% Female) CRC patients that were age 65 or older with a mean Charlson Comorbidity Index (CCI) of 2.30 (± 3.26) were identified by ICD-9 code 153.x and/or 154.x. The overall cost for patient groups with different characteristics was compared by using standard t-test, Wilcoxon test or ANOVA. The cost was further modeled in a generalized linear model (GLM) with a log-link and gamma distributed variance functions. RESULTS: 38% of CRC patients had surgery, 6.0% conducted radiotherapy and 5.7% received chemotherapy. Patients on chemotherapy were incurred with the highest cost ($41,867), followed by radiotherapy ($13,812) and surgery ($4,964). Patients on 5-FU, Leucovorin, Oxaliplatin and Bevacizumab (5-L-O-B) had the highest estimated cost ($52,158), as its patient population was the most severe (Mean CCI=7.6), followed by 5-FU, Leucovorin and Oxaliplatin (5-L-O) ($39,435, Mean CCI=4.6). The average cost for other chemotherapies was $30,838. Most colon cancer patients (2,761 patients) used the drug combination of 5-L-O and 5-L-O-B (1,450 patients). Oxaliplatin and Bevacizumab were administered at a lower dose when used in combination with other drugs than used alone.  About 60% of the patients received these combinations as adjuvant therapy after surgery. CONCLUSIONS: This study provided information on the average annual cost of elderly CRC patients by treatment type and disease comobidity. The analysis illustrated the utilization of chemotherapy in CRC treatment: the common drug combinations and its costs, dosing and administration information as well as chemotherapy use after surgery.