Background: On February 12 and 26, 2004, the FDA approved bevacizumab and cetuximab, respectively, two biologic drugs for the first–line treatment of metastatic colorectal cancer (mCC), followed by panitumumab, another biologic approved on September 27, 2006. NCCN guidelines suggest that biologics may be added to fluorouracil and leucovorin (5-FU/LV), irinotecan (IRI), or oxaliplatin (OX), yet a number of recent articles raise questions regarding the added benefit of biologics on top of a backbone chemotherapy regimen, which might affect the utilization pattern of biologics. To examine the adoption of biologics, we document the rate of treatment with biologics in addition to backbone chemotherapy as first-line treatment (Tx1) of mCC. Methods: This study uses the Surveillance, Epidemiology and End Results (SEER)-Medicare data and examines mCC patients diagnosed in 2003-2007 who received chemotherapy with or without biologics as Tx1. Due to the small sample size of cases involving the combined use of irinotecan and oxaliplatin (IROX) as Tx1, IROX patients were excluded from this study. The remaining 2995 observations are broken down by regimen and by year of Tx1 initiation (Table). Results: The use of biologics (90% of which was bevacizumab) in addition to chemotherapy as part of Tx1 exploded from 9% in 2004 to 55% in 2005. Utilization was highest (62%) in 2006 and fell slightly in 2007 to 57%. Of all cases utilizing biologics, 67% were oxaliplatin-based regimens, 19% were fluorouracil-based regimens and 15% were irinotecan-based regimens. Conclusions: In the first few years following FDA approval, the use of biologics in addition to backbone chemotherapy increased rapidly as first-line treatment of elderly Medicare metastatic colon cancer patients, leveling out at around half of all patients receiving chemotherapy. Biologics use has been most common among patients treated with oxaliplatin.