Study Design. A survey of occupational risks for low back trouble in two police forces discordant for one known physical stressor (wearing body armor weighing approximately 8.5 kg). Objective. To determine the hazard for first- onset and subsequent course of low back trouble associated with occupational physical and psychosocial stressors. Summary of Background Data. Various occupational physical stressors have been associated with the prevalence of back pain, but their relationship with first-onset low back trouble is uncertain. Psychosocial factors reportedly ere important determinants of chronicity. Methods. Anamnestic data on low back trouble were collected from representative random samples of 'exposed' and control forces, along with variables describing exposure to occupational physical stressors and sports pursuits. Psychometric tests were administered. Results. Occupational risk factors for first-onset low back trouble were determined from lifetables based on officers with no previous back pain history. Survival time to first- onset was effected adversely by wearing body armor and, less so, by vehicular exposure and sports participation. The proportion with persistent (chronic) trouble did not depend on length of exposure since onset, but longer service was associated with recurrent episodes. Chronicity was related to distress and blaming police work. Work loss was associated with blaming work and wearing body armor. Changing to lighter duties after development of low back trouble occurred rarely. Conclusions. Exposure to occupational physical stress seems detrimental; it reduced survival time to first-onset of low back trouble. Recurrence was associated with time since onset, but persistent trouble was not. Sports participation was a risk if occupational hazards were high.