A retrospective follow-up study on 924 physical education (PE) and 3,239 language (L) teachers was performed to study whether life-long high physical activity affects the risk of breast cancer. The Finnish Cancer Registry found 128 malignant breast cancers among these women in a follow-up during 1967-1987. The standardized incidence ratio (SIR) for all PE teachers was 1.28 (n.s.) and for L teachers 1.59 (p less than 0.001). Before the menopause (below age 50) SIR for PE teachers was 0.93 (n.s.) and for L teachers 1.51 (p less than 0.05). These results suggest that before menopause the risk of breast cancer in physically active PE is smaller than in the less active L. A Poisson regression analysis, taking into account the reproductive factors together with age and observation period, did not show any significant difference between PE and L teachers, probably due to the relatively small number of cases (n = 22) in the PE teacher group.