This study examined the relation between return to work and the maintenance of treatment gains made over the course of a rehabilitation intervention. The study sample consisted of 110 individuals who had sustained whiplash injuries in rear collision motor vehicle accidents and were work-disabled at the time of enrolment in the study. Participants completed pre- and post-treatment measures of pain severity, disability, cervical range of motion, depression, posttraumatic stress symptoms, and catastrophizing. Pain severity was assessed again at 1-year follow-up. At 1-year follow-up, 73 participants had returned to work and 37 remained work-disabled. Analyses revealed that participants who returned to work were more likely to maintain treatment gains (77.5%) than participants who remained work-disabled (48%), χ = 6.3, P < 0.01. The results of a regression analysis revealed that the relation between return to work and the maintenance of treatment gains remained significant (β = 0.30, P < 0.01), even when controlling for potential confounders such as pain severity, restricted range of motion, depression, and pain catastrophizing. The Discussion addresses the processes by which prolonged work-disability might contribute to the failure to maintain treatment gains. Important knowledge gaps still remain concerning the individual, workplace, and system variables that might play a role in whether or not the gains made in the rehabilitation of whiplash injury are maintained. Clinical implications of the findings are also addressed.