OBJECTIVES: Standard therapy for chronic lymphocytic leukaemia (CLL) is fludarabine, followed by alemtuzumab. Patients resistant to both treatments (double refractory (DR)-CLL) receive best supportive care (BSC). Ofatumumab is licensed for the treatment of DR-CLL, but was not recommended by the National Institute for Health and Care Excellence (NICE technology appraisal 202 (TA202)). There is little clinical data for BSC in DR-CLL. TA202 identified a single-arm study of ofatumumab and an observational study of BSC. Survival for BSC in TA202 was modelled by fitting a Weibull distribution to data for ofatumumab non-responders. The effects of ofatumumab were estimated by assuming a proportional hazards model.  The objective of this study was to investigate the potential sources of survival data for BSC, and the impact of the choice of survival distributions used in TA202 on the cost-effectiveness of ofatumumab. METHODS: Individual patient-level data was reconstructed from Kaplan-Meier curves using a published algorithm. Plausible survivor functions were fitted to the data using Markov chain Monte-Carlo simulation. Goodness-of-fit to the observed data was assessed using deviance information criterion and the clinical plausibility of extrapolations using subjective opinion. The cost-effectiveness model from TA202 was reproduced including alternative survival data. RESULTS: A Weibull distribution provided the best fit to the data and was most clinically plausible. If proportional hazards were assumed (as in TA202) the incremental cost-effectiveness ratio (ICER, incremental cost per life year gained) was £52,400, similar to £49,252 reported in TA202. Relaxing the assumption of proportional hazards increased the ICER to £85,618. CONCLUSIONS: The best fitting and most clinically plausible model was a Weibull distribution, giving results consistent with TA202; however both the use of a subgroup of a single arm trial and of data from multiple studies has a high level of uncertainty. Relaxing the assumption of proportional hazards increased the ICER.