Background. Although up to 80% of women can return to work after treatment for breast cancer, maintaining an affiliation to the labour market may be a challenge, as shown by the fact that the risks for unemployment and early retirement are increased in the years after treatment of cancer. It is important to understand the work problems experienced by cancer survivors, including their ability to work. The aim of this study was to determine whether the ability of long-term breast cancer survivors to work was different from that of a cancer-free control group. Material and methods. In this population-based cross-sectional questionnaire study, 776 breast cancer survivors were matched with 1552 cancer-free women. Women with breast cancer diagnosed in 1997-2000 were identified in the Danish Cancer Registry, and the cancer-free controls were sampled from the Central Population Registry. Work ability was measured from a single question on the 'work ability index'. Furthermore, the questionnaire contained questions on socioeconomic factors, health-related factors and factors related to the workplace. Results. The overall response rate was 57% (493 survivors and 830 controls). After exclusions, the study population consisted of 170 survivors and 391 controls. Women with a diagnosis of breast cancer who had survived at least five years and had returned to work reported significantly poorer work ability than cancer-free controls. In models with adjustment for socioeconomic factors, health-related factors and support at work, the factors most strongly associated with impaired work ability were low income, fatigue and little help and support from a supervisor. Conclusion. Our findings indicate that the work ability of long-term breast cancer survivors who are disease-free and back in work is impaired in comparison with that of cancer-free women.