OBJECTIVES: Idelalisib/ rituximab (IR) is licenced for the treatment of adults with CLL who either have received at least one previous therapy and as first line treatment for patients with del17p/TP53 mutations. Prior to the availability of IR, individuals in these patient groups received best supportive care (BSC). The clinical efficacy of the IR in these patient groups was demonstrated in a Phase III RCT (‘study 116’). The cost-effectiveness of IR in this patient group is unknown. METHODS: A response stratified partitioned survival model (overall survival - OS, progression free survival - PFS) was developed to estimate the lifetime costs and benefits associated with IR and BSC for a Scottish NHS perspective using a lifetime horizon and monthly cycles. OS, PFS, overall response (OR) and resource use data was taken directly from study 116. Information from study 116 was used as far as possible for patients with del17p / TP53 mutations, with expert opinion used where necessary. Utility scores were taken from published sources. Unit/drug costs were taken from national databases and discounted at 3.5% p.a. Probabilistic and deterministic sensitivity analyses were conducted to estimate the confidence around the results. Outcomes are reported via incremental cost-effectiveness ratios (ICER, benefit expressed as QALYs). RESULTS: For all patients the ICER for all patients was £32,180/ QALY (ΔQALYs:2.04, ΔCosts:£64,629). In patients with del 17p/TP53 mutations the ICER was £19,040/QALY (ΔQALYs:4.39, ΔCosts:£83,636). The results were sensitive to changes in OR rates and utility values. In particular, the ICERs fell below £30,000/QALY if utility values from previous UK HTAs of treatments for CLL were used. The ICERs were robust to changes in adverse event rates/costs and alterations to background resource use patterns. CONCLUSIONS: IR is likely to be a cost-effective intervention in all CLL patients for which it has achieved European marketing approval.