Regular rest breaks are recommended to prevent accumulation of accident risk during sustained activities. We examined the effect of rest breaks on temporal trends in industrial accident risk, by assessment of accident records from a large engineering company, obtained over 3 years. In 2 h of continuous work, relative risk of an accident in the last half-hour of a shift was 2.08 (95% CI 1.73-2.43) higher than in the first half-hour. Trends in risk did not seem to differ between three 2-h work periods. Regular rest breaks seem to be an effective way to control accumulation of risk during industrial shift-work.