Observational studies of the effect of beta-interferon (IFNbeta) on accumulation of fixed disability in relapsing remitting multiple sclerosis (RRMS) in clinical practice have been difficult to interpret due to bias. The aim of this study of 175 RRMS patients was to use Bayesian analysis to establish whether IFNbeta attenuates disability relative to a cohort of matched historical control subjects from the Sylvia Lawry Centre for MS Research. A sensitivity analysis was based on a range of prior probability distributions for IFNbeta efficacy derived from a published meta-analysis of randomised controlled trials (RCTs) of IFNbeta, and the data were interpreted both unmodified and using variance inflation and point estimate bias correction; the corrected data interpreted in the light of the most likely prior probability distribution yielded a 95 % posterior credible interval for the odds ratio of accumulation of fixed disability after two years of IFNbeta therapy of 0.52, 0.94. It is concluded that two years of IFNbeta therapy for RRMS reduces accumulation of fixed disability in clinical practice.