The aims of this thesis were to investigate the long-term effects of adjuvant tamoxifen treatment on breast cancer recurrence and mortality, cardiovascular disease, and the incidence of secondary cancer. Between 1982 and 1992, postmenopausal patients with early stage breast cancer were included in a randomized clinical study of 2 or 5 years of postoperative tamoxifen therapy. The trial was planned by the Swedish Breast Cancer Group, and it included 4610 patients. Follow-up on causes of death, hospitalizations and secondary cancers were obtained from national population-based registries.  All-cause mortality, breast cancer-specific mortality and mortality from coronary heart disease were decreased in the 5-year group, but the incidence of endometrial cancer was increased (Paper I). The incidence and mortality of cerebrovascular diseases were increased during the active treatment phase, and reduced after the active treatment (Paper II). Similar results were seen for subgroups of cerebrovascular diseases such as stroke and ischemic stroke. In the 5-year group, the morbidity from coronary heart disease was reduced during treatment but not after treatment was stopped (Paper III). This was the case also for heart failure and for atrial fibrillation/flutter. For secondary cancers the lung cancer risk was reduced, as well as the lung cancer mortality (Paper IV). An increased risk was observed for endometrial cancer, but appeared to decrease over time. The risk of contralateral breast cancer was reduced, with most of the reduction after treatment was stopped. For distance recurrences the risk was reduced both during treatment and a few years after treatment was stopped. The breast cancer mortality was also reduced, especially during the post-treatment phase.