Study Design. A cohort of 1848 workers, representative of all sectors of industry, who were compensated for a low back injury in 1988 but not in the previous 2 years, was followed over 24 months. Objectives. To determine the prognostic value of the physician's initial diagnosis of back problems. Summary of Background Data. In the absence of a standardized classification of diagnoses of back pain, this study aimed to provide an element of validity to a classification previously proposed that consists of ''specific'' and ''nonspecific'' back pain. Methods. Medical charts were reviewed at the Quebec Worker's Compensation Board to extract the diagnosis made by the treating physicians within 7 days of the first day of absence from work. Diagnoses were categorized into ''specific'' (lesions of vertebrae and discs) and ''nonspecific'' (pain, sprains, and strains). The history of compensated work absence for low back pain in the following 24 months was obtained. Results. A specific diagnosis was found in 8.9% (165) of the workers, accounting for 31.0% of the patients who accumulated 6 months or more of absence in 2 years. Increasing age and daily amount of compensation also were associated with an increased risk of chronicity. Conclusions. The physician's initial diagnosis was highly associated with the risk of chronicity. The explanation for this result is complex, involving the nature of the underlying lesion as well as the impact of the diagnosis ''label'' on the worker and on the physician-patient relationship.