The study investigated the relationship between motivation, self-efficacy and demographic variables, and determined if they affect the performance of health promotion behaviours in overweight or obese middle-aged American women. The sample consisted of middle-aged American women from a small town in Michigan. Two groups of women aged 30–65, one with a body mass index (BMI) range of 25–29, and the second with a BMI of ≥ 30, completed the Health Self-Determinism Index, the General Self-Efficacy Scale and the Health Promoting Lifestyle Profile-II. Self-efficacy was found to be a significant predictor in the performance of health-promoting behaviours in both the overweight and obese participants in this study. Motivation was not found to be a significant predictor of performance of health promotion behaviours in either participant group. Education was found to be a significant predictor of performance of health promotion behaviours in the obese participants only. Nurses need to develop effective methods of supporting self-efficacy in both the overweight and obese middle-aged American women. Nursing-based research focusing on interventions to improve self-efficacy, as well as studies identifying effective educational techniques to improve the practice of health promotion behaviours in this population is necessary. Further investigation into the effect of motivation and demographics on the performance of health promotion behaviours is also needed.