The incidence of sciatic pain among 25-49 year-old men in three different types of work: machine operating, dynamic physical work (construction carpenters), and office work was assessed prospectively. Postal questionnaires both at the beginning and the end of the 3-year follow-up were answered by 1149 men who at baseline had no history of sciatic pain. The crude risk ratio of the incidence of sciatic pain was 1.6 (95% confidence interval 1.2-2.2) for the machine operators and 1.7 (1.3-2.4) for the carpenters when the office workers were referents. The adjusted risk ratios were 1.4 (1.0-1.9) and 1.5 (1.1-2.1), respectively. Previous history of severe lumbago or other low-back pain increased the risk fourfold. Frequent physical exercise and smoking were of borderline significance as predictors.