Introduction– During the extended course of multiple sclerosis (MS) there are ample opportunities for the patients to develop other illnesses including cancer, a potential long-term complication of the immunosuppressive drug treatment in MS. Material and methods– A retrospective cohort study was done to estimate the relative risk of cancer in a population of MS-patients from three Norwegian counties by record linkage with the Cancer Registry of Norway. The cohort comprised 1271 MS-patients, 530 men and 741 women, identified in five longitudinal population-based incidence studies. The reporting of cancer cases has been compulsory in Norway since 1952. Results– We found 73 cancer cases (standardized incidence ratio (SIR) = 0.86, 95% CI 0.68–1.09). Mean follow-up time was 18.4 years. A significant excess of breast cancers was observed (SIR = 1.70, 95% CI 1.05–2.60). A non-significant excess of cancer in the urinary tract was observed. No significantly increased risk in hematologic and lymphatic malignancies or malignant brain tumors was observed. The incidence of cancer of the digestive organs was significantly lower than expected (SIR = 0.51, 95% CI 0.24–0.93). Conclusion– Overall, the study indicates that an MS-patient is not at any unusual risk for subsequent development of cancer compared to the normal population.