OBJECTIVES: Life extending therapies (LETs) for metastatic castrate-resistant prostate cancer (mCRPC) increase overall survival. This study examined adherence to LETs and concomitant corticosteroids, and healthcare costs. METHODS: A retrospective claims study of commercial and Medicare Advantage enrollees with evidence of prostate cancer (ICD-9: 185.xx) between July 1, 2006 – June 30, 2011. mCRPC patients were identified based on evidence of LET use: docetaxel (DOC), cabazitaxel (CAB), and/or abiraterone acetate (ABI). The index date was the first date of chemotherapy. Patients were continuously enrolled for 6 months before (baseline) and ≥6 months after the index date until December 31, 2011 or death (follow-up). Adherence was defined as the proportion of days covered (PDC). Costs were the sum of health plan- and patient-paid amounts. Descriptive statistics summarized PDC and costs during follow-up. RESULTS: A total of 1,198 patients had ≥1 LET (DOC: 1,196, CAB: 27, ABI: 109). Mean±SD age was 69±9 years. Average PDC for LET was: DOC: 0.91, CAB: 0.88, ABI: 0.96. Half or more had a concomitant corticosteroid: DOC: 552, CAB: 17, ABI: 99, with an average PDC of: DOC 0.57, CAB 0.62, ABI 0.74. Average±SD cumulative 12-month healthcare costs were $76,550±$82,485.  Average±SD per-patient-per-month (PPPM) healthcare costs during LET were higher for patients with a concomitant corticosteroid than those without: $9,307±$7,436 vs. $5,929±$11,103; p<0.001.  Patients with high adherence to a concomitant corticosteroid (PDC≥80%) had higher average PPPM costs during LET than patients with lower adherence (PDC<80%) or without: $9,028±$7,133 vs. $7,339±$9,992; p=0.002. CONCLUSIONS: mCRPC patients had high adherence to LET but lower adherence to indicated concomitant corticosteroid. Average PPPM costs were higher for patients with greater adherence to concomitant corticosteroids than patients with lower adherence or no use. Multivariate analyses are planned to better understand the association between corticosteroid adherence during LET use and costs. The database timeframe did not allow for the inclusion of newer therapies.