In an investigation of the effect of heavy physical work on the back 217 concrete reinforcement workers aged 25-54 and a reference group of 202 house painters of similar age were interviewed about their back symptoms. Data on occupational history, accidents, and leisure time activities were collected with a questionnaire. The cumulative incidence rate of sciatic pain was significantly higher among the reinforcement workers than the painters. As regards the occurrence of lumbago and non-specific back pain, however, the groups were alike. In both occupational groups sciatic pain during the previous 12 months was associated with earlier back accidents (odds ratio 2.8, 95% confidence interval 1.8-4.5). The accident rate of the reinforcement workers was higher than that of the painters, and this difference seemed to explain their higher rate of sciatic pain.