OBJECTIVES: This multicentered randomized controlled trial evaluated the effect of worksite exercise intervention on perceived work ability and sick leaves. METHODS: Women (N=260, mean age 40 years) engaged in physically demanding laundry work were individually randomized into an intervention (N=133) or control (N=127) group. Perceived work ability was assessed with questionnaires at 3, 8, 12, and 15 months. Sick leave information was obtained from the personnel administration. Follow-up attendance was 100% at 3 months but declined gradually to 90% by 15 months. Both the intervention and control subjects received a 30-minute feedback on their physical capacity from a physiotherapist and individual exercise prescription and counseling. The intervention subjects also participated in worksite exercise training guided by a physiotherapist. Sixty-minute sessions (N=26) were held once a week for 8 months. About 50% of the intervention group participated in at least two-thirds of the sessions. RESULTS: According to a dichotomized work ability index, at 12 months, workers with "good" or "excellent" work ability increased more in the intervention group than in the control group (11.0%, 95% CI 0.2-21.9), as did the health-related prognosis of work ability at 8 months (8.1%, 95% CI 0.5-16.3). There were no statistically significant differences between the two groups as regards job satisfaction, work ability index (including series of questions on 7 items), or sick leaves. CONCLUSIONS: Physical activity once a week at worksites improves the perceived work ability of women with physically demanding work only slightly. Perceived work ability and sick leaves cannot be affected very positively using single-component exercise intervention. Work ability promotion may need a more multiprofessional approach.