Background <p>Early physical therapy for workers reporting low back pain (LBP) may reduce disability and improve return to work. This study aimed to explore the relationship between the timing of physical therapy commencement and the duration of work disability after the onset of compensable LBP.</p> Methods <p>We conducted a retrospective cohort analysis of workers with workers’ compensation claims for LBP in two Australian states. We investigated the association between the timing of physical therapy commencement and work disability duration using an accelerated failure time model. Median duration of work disability in paid calendar weeks was the principal outcome.</p> Results <p>We examined 9160 accepted workers’ compensation claims for LBP. Patients who had not seen a physical therapist had the shortest duration of disability (median, 4.1 weeks). In those who had seen a physical therapist, the median duration of work disability was associated with the timing of commencement of physical therapy, from 8.0 weeks for care within 7 days of the injury to 34.7 weeks when care was commenced greater than 30 days after the onset of injury. Our adjusted model demonstrated that, compared to physical therapy within 7 days of injury onset, commencement of physical therapy between 8 and 14 days, 15 and 30 days, and greater than 30 days was associated with a 37.0% (Time ratios (TR) 1.37; 95% CI (1.23, 1.52)), 119% (TR 2.19; 95% CI (1.96, 2.44)) and 315% (TR 4.51; 95% CI (4.06, 5.02)) increased likelihood of longer disability duration, respectively.</p> Conclusions <p>In workers with work-related LBP undertaking physical therapy, early commencement of physical therapy was associated with a significantly shorter duration of disability. Although we cannot establish causality, our findings highlight the potential benefits of initiatives that promote timely initiation of treatment in reducing extended work disability for injured workers undergoing physical therapy for LBP.</p>