Patients with non-insulin-dependent diabetes mellitus (NIDDM) were advised to comply with a complex behavioral regimen of diet and exercise. The relationship between social support satisfaction and social support network size was evaluated using the Social Support Questionnaire for 32 men and 44 women with a confirmed diagnosis of NIDDM. Control of diabetes, as measured by the glycosylated hemoglobin assay, was significantly correlated with social support satisfaction for women but negatively correlated with social support satisfaction for men. Social support network size differentially predicted success in a program for men and women. For women, network size was significantly correlated with failure to attend sessions and with failure to complete a diary. For these women, network size was not significantly correlated with weight loss, which was the goal of the program. For men, network size was correlated with increases in weight, cholesterol, and triglycerides over an 18-month period. We conclude that social support network size and satisfaction have different functions for men and women faced with a serious chronic illness. Network size adversely affects success in a program, whereas social support satisfaction has some benefits for women. The direction of the influence of social network may be determined by the similarity or dissimilarity of network norms to the desired behavior.