Changes in coronary risk factors, health behaviours, and health-related quality of life (HRQOL) were examined by tertiles of social support group attendance in 440 patients (21% females) with coronary artery disease. All patients participated in the Multicenter Lifestyle Demonstration Project (MLDP; eight hospital sites in the USA), an insurance-covered multi-component cardiac prevention program including dietary changes, stress management, exercise and group support for 1 year. Significant improvements in coronary risk factors, health behaviours, and HRQOL were noted at 1 year. Several of these improvements (i.e. systolic blood pressure, health behaviours, HRQOL) were related to social support group attendance, favoring those who attended more sessions. The associations between support group attendance to systolic blood pressure and to four HRQOL subscales (bodily pain, social functioning, mental health, and the mental health summary score) remained significant when controlling for changes in health behaviours, but dropped to a non-significant level for the HRQOL subscales 'physical functioning', 'general health' and 'role-emotional'. These results suggest an independent relationship of social support group attendance to systolic blood pressure while improvements in quality of life may be in part due to improved health behaviours facilitated by increased social support group attendance.