OBJECTIVES: Alemtuzumab has been demonstrated to reduce the amount of malignant lymphocytes in patients with chronic lymphocytic leukemia (CLL). The current study aimed to quantify the incremental cost to Medicare of treating CLL patients with alemtuzumab. METHODS: Claims records (1999-2007) from the Medicare 5% national sample were analyzed. Patients with continuous enrollment for ≥3 months prior to their first observed claim with a CLL diagnosis, ≤2 malignancies, and ≥1 claim for alemtuzumab were included. A pre-post design was used to quantify the incremental costs associated with alemtuzumab by calculating health care costs within 6 months after alemtuzumab initiation relative to the 6-month period before alemtuzumab initiation. Mean monthly (per-patient per-month, PPPM) costs were calculated and were grouped by sites of care, service type, tests and procedures, treatment and drugs, and by adverse events. Statistical comparisons between the pre- and post-treatment periods were based on paired Student t-tests. RESULTS: A total of 81 CLL patients treated with alemtuzumab formed the study population. The mean age was 75.2 years and females represented 38.3%. Patients were observed for an average of 50 months and mean time between the first CLL diagnosis and initiation of alemtuzumab treatment was 36 months. After alemtuzumab initiation, mean total health care costs increased from $4,272 to $10,385 PPPM (P<0.0001). Patients had a mean of 11.8 claims for alemtuzumab and the mean cost for alemtuzumab was $4,006 PPPM or 39% of total costs. PPPM costs associated with diagnostic codes for cytopenia, infection, and cardiac dysfunction were greater during the post- compared with the pre-alemtuzumab period (cytopenia: $1,658 vs. $4,114; infection: $107 vs. $841; cardiac dysfunction: $766 vs. $1,692; P<0.05 for all). CONCLUSIONS: Amongst a cohort of Medicare fee-for-service patients with CLL, alemtuzumab was associated with a significant increase in healthcare costs in the 6 months after initiation of therapy.