OBJECTIVES: It is widely thought that adherence rates to intravenous (IV) chemotherapy regimens for colon cancer are high.  However, there are no known formal assessments of this issue. METHODS: A retrospective analysis was performed using the Optuminsight Oncology claims database. Patients aged 18 years and older, diagnosed with CRC between July 1, 2004 and December 31, 2010, who were insured by a commercial health plan were included in the study.  Adherence to the following IV chemotherapy regimens was assessed using the National Comprehensive Cancer Network (NCCN) guidelines as the standard for expected cycle/regimen duration: FOLFOX, FOLFOX+bevacizumab, FOLFIRI, and FOLFIRI+bevacizumab.  Adherence was assessed using the medication possession ratio (MPR), calculated as the number of days a patient was covered by their chemotherapy regimen, according to NCCN guidelines,  divided by the number of days elapsed from the first to the last infusion of that regimen. RESULTS: A total of 46,941 chemotherapy cycles in 6,880 patients were analyzed.  Overall, adherence rates to IV chemotherapy was fairly high, with mean MPR ranging between 0.84 and 0.88 for these regimens.  However, a substantial proportion of patients for each regimen experienced low adherence.  Twenty five percent of patients receiving FOLFOX,  FOLFOX+bevacizumab, and FOLFIRI+bevacizumab regimens experienced MPR<0.8.  Additionally, approximately 35% of patients receiving FOLFIRI experienced an MPR<0.8.  At least 10% of patients receiving FOLFOX regimens had an MPR less than 0.7; while at least 10% of patients receiving FOLFIRI regimens had an MPR of less than 0.6. CONCLUSIONS: Although overall rates of adherence were fairly high, a substantial subpopulation experienced low adherence to each of these IV regimens per NCCN guideline recommendations.  The reasons for the low adherence rates need to be explored as this could have an impact on efficacy.  These results also highlight the drawback of relying solely on summary statistics at the population level.