Objectives Recent research has suggested a moderate link between night work and breast cancer in women, mainly through case–control studies, but non-significant studies are also common and cohort studies are few. The purpose of the present study was to provide new information from cohort data through investigating the association between the number of years with night work and breast cancer among women.

Design Cohort study of individuals exposed to night shift work in relation to incidence of breast cancer in women.

Setting Individuals in the Swedish Twin registry, with follow-up in the Swedish Cancer Registry.

Participants 13 656 women from the Swedish Twin Registry, with 3404 exposed to night work.

Outcome measures Breast cancer from the Swedish Cancer Registry (463 cases) during a follow-up time of 12 years.

Results A Cox proportional hazards regression analysis with control for a large number of confounders showed that the HR was HR=1.68 (95% CI 0.98 to 2.88) for the group with >20 years of night work. When the follow-up time was limited to ages below 60 years, those exposed >20 years showed a HR=1.77 (95% CI 1.03 to 3.04). Shorter exposure to night work showed no significant effects.

Conclusions The present results, together with previous work, suggest that night work is associated with an increased risk of breast cancer in women, but only after relatively long-term exposure.