Context: There are limited studies describing mortality and morbidity in patients treated for hyperthyroidism, and no data exist for people with treated hypothyroidism. Objective: The objective of the study was to describe all-cause mortality and vascular mortality and morbidity in patients after treatment for hyperthyroidism and hypothyroidism. Design: This was a population-based cohort study from 1994 to 2001. Setting: The study was conducted in Tayside, Scotland. Patients: All patients were treated for thyroid dysfunction. Intervention(s): Event rates among patients with thyroid dysfunction were compared with rates in the general population. We measured standardized mortality ratio and standardized incidence ratio (SIR). Main Outcome Measure(s): The primary outcome was all-cause mortality. The secondary outcome was serious vascular event, the composite end point of nonfatal myocardial infarction, nonfatal stroke, or vascular death. Results: There were 15,889 primary hypothyroid and 3,888 hyperthyroid patients. There were 3,116,719 patient-years of follow-up in 524,152 subjects in the general population. No increase was found in all-cause mortality or serious vascular events in patients with treated hypothyroidism or hyperthyroidism. Nonfatal ischemic heart disease [SIR 1.23, 95% confidence interval (CI) 1.10–1.36] and dysrhythmias (SIR 1.32, 95% CI 1.11–1.57) were increased in treated hypothyroidism when adjusted for age, sex, diabetic status, and previous vascular disease. In treated stabilized hyperthyroidism, only the risk of dysrhythmias was increased (SIR 2.71, 95% CI 1.63–4.24). Risk of heart failure or cerebrovascular disease was not increased in either patient group. Conclusions: We found no increase in all-cause mortality in subjects with treated thyroid disease. However, there was increased risk of cardiovascular morbidity in patients with treated primary hypothyroidism and dysrhythmias in treated hyperthyroidism.