BACKGROUND: People, who are single, have a blue-collar job or low income have an increased cardiovascular risk. This study on myocardial infarction sought to explore whether the socio-economic pattern of disease has any relationship with obesity. METHODS: In the cohort are 20,099 middle-aged men of whom 9,150 were manual and 9,190 nonmanual workers and 1,759 were self-employed. A total of 4,081 were single, 16,018 cohabiting. The body mass index (BMI) cut-off values for overweight and obesity were 25-30 and >/=30 kg m-2, respectively. Local and national registers were used to monitor incidence of events over 18 years. RESULTS: Obesity was associated with an increased incidence of coronary events and deaths in each occupational group. Being single significantly increased the risk associated with obesity. After stratification for civil status the risk associated with obesity was limited to those who were single and who either had a blue-collar job or were self-employed. The multivariate-adjusted relative risk (RR) of coronary events and deaths in obese manual workers who were single was 1.91 (95% confidence interval: 1.21-3.02) and 2.54 (1.74-3.69), respectively, times higher than it was amongst those who were cohabiting. Amongst those who were self-employed, the corresponding age-adjusted RRs were 4.79 (1.69-13.57) and 3.80 (1.62-8.93). CONCLUSIONS: Adjusted for lifestyle and biological risk factors, the increased risk of coronary events and death for obese men with manual jobs was applicable only to those who were single. It is concluded that being single significantly increases the cardiovascular risk associated with obesity.