Objective: This study provides updated, in-depth estimates of colorectal cancer (CRC) treatment costs. Methods: This retrospective cohort study included patients aged =65 years, who were recently diagnosed with colon (CC) or rectal (RC) cancer in a SEER registry between 1996 and 2002 (n=60,916) and 1:1 matched (by age, sex, geographic region) non-cancer comparison patients from a 5% Medicare sample. We assigned costs to phases as follows: 1) initial: costs in the period up to one year after diagnosis among patients with =13 months survival; 2) continuing: costs in the years between the initial and terminal years among patients with =36 months survival; and 3) terminal: costs in the final year of life. Terminal costs were assigned first (all costs considered terminal for patients who lived <13 months). Costs reflect all provider payments for cancer patients in excess of those for matched comparison patients (2006 US Dollars). Results: Cancer-related CRC costs averaged $32,303 in the initial phase, $3,548 per year in the continuing phase, and $14,323 in the terminal phase. Initial-phase costs were similar by site (CC: $32,528; RC: $31,701; P=0.015), while continuing-phase costs were roughly 1/3 higher for RC versus CC ($4,266 vs. $3,287; P<0.001). Terminal-phase costs were $14,197 for CC and $14,654 for RC (P=0.424). Initial-phase CC costs were $17,278 and $40,501 for Stages 0 and 4, respectively (P<0.001), compared to $14,060 and $37,235 for Stage 0 and 4 RC (P<0.001). Continuing costs ranged from $2,499–$17,861 and $2,822–$17,741 for Stage 0–4 CC and RC patients, respectively; terminal costs ranged from $7,814–$27,742 for Stages 0–4 CC and $6,376–$20,047 for Stages 0–4 RC patients. Conclusion: Excess costs associated with CRC are striking and vary considerably by treatment phase, cancer site, and stage at diagnosis. Interventions aimed at earlier diagnosis and prevention have the potential to reduce cancer-related health care costs.