6119 Background: IM (Glivec, Gleevec) is an oral targeted therapy with unprecedented efficacy in CML and GIST. Prescription compliance and persistency of pts receiving IM were measured by analysis of pt-level pharmacy claims data. Methods: Compliance and persistency were determined by analyzing the prescription-filling activity of pts (N=4043) compared with the prescribing activity of their physicians (N=3316) using pt pharmacy records accrued over 24 months (1/03–12/04). Observed average daily consumption (DACON) and average prescribed days of therapy were calculated and compared. Also, compliance and persistency were examined by pt demographics and initial IM dose prescribed. Results: Overall compliance (defined as medication possession ratio = apparent mg taken/mg prescribed) was 75%, with CML pts showing slightly greater compliance (78%) than GIST pts (73%). Fifty percent of pts were 100% compliant, the greatest compliance being found in pts initially treated with IM 300 or 400 mg/day (77%). Persistency (time on therapy without significant gaps in refills) averaged 255 days over 24 months. The most persistent pts were those initially given 300 or 400 mg/day (13.0 and 12.9 months, respectively). DACON was 400 mg/day for 65% of patients, but fluctuated above and below 400 mg/day in 18% and 17% of pts, respectively. Conclusions: This is the first assessment of pt compliance and persistency with prescribed IM therapy. Although less pronounced than with most other non-oncology products, suboptimal compliance and persistency with IM are a concern as doses <300–400 mg may result in plasma levels lower than needed to eliminate cancer cells. Patient support programs and improved communication on the importance of adhering to recommended dosing could potentially optimize outcome and further reduce risk of relapse and progression. [Table: see text]