The effects of marital status and change in marital status on mortality among middle-aged British men were examined in a prospective cohort study, the British Regional Heart Study. This is a nationally representative cohort of men selected at random from general medical practices in 24 towns in England, Wales, and Scotland. It comprises 7,735 men aged 40-59 recruited in 1978-1980 and followed up for 11.5 years. Marital status and a wide range of biologic and lifestyle variables were measured at screening, and changes in marital status were assessed after 5 years. Single (never-married) men had an increased risk of cardiovascular disease mortality (relative risk (RR) = 1.5, 95% confidence interval (CI) 1.0-2.2) and noncancer, noncardiovascular mortality (RR = 1.8, 95% CI 1.1-3.3) after adjustment for potentially confounding variables: age, social class, smoking, recall of ischemic heart disease, recall of diabetes mellitus, use of antihypertensive drugs, body mass index, physical activity, alcohol intake, employment status, systolic blood pressure, blood cholesterol, and forced expiratory volume in 1 second. Divorced/separated men were not at increased risk of mortality, and widowed men were only at increased risk of other non-cardiovascular disease mortality (RR = 2.4, 95% CI 1.1-5.3). There was no effect of marital status on cancer mortality. Men who divorced during the follow-up period were at increased risk of both cardiovascular disease mortality (RR = 1.9, 95% CI 0.9-3.9) and other non-cardiovascular disease mortality (RR = 4.0, 95% CI 1.5-10.6), but men who became widowed during this time were not at increased risk. The excess mortality among single and recently divorced men was not explained by poor health or by exposure to a wide range of risk factors. It is unlikely that selection bias, chance, or artifact is responsible for the general relation between marital status and mortality. Variable and incomplete control for confounding by socioeconomic status and risk factors for common diseases may explain some of the inconsistencies observed between studies and between different categories of unmarried men (i.e., never-married, widowed, and divorced). It is possible that the social support offered by marriage exerts a protective effect for some men.