Purpose: To identify factors influencing quality of life based on health-promoting behavior in patients with chronic illness. Design and Methods: This descriptive study was conducted over 8 months in 2 001 on 1, 748 chronic adult outpatients at a university hospital in Seoul, Korea. Study variables and instruments were health perception, perceived benefits of action, perceived barriers to action, self-esteem, self-efficacy, social support, commitment of planned action, activity-related affect, health-promoting behavior, and quality of life. Results: The data indicated a statistically significant positive correlation among health-promoting behavior, perceived benefits of action, self-efficacy, social support, self-esteem, health perception, activity-related affect, and quality of life. A statistically significant negative correlation was found between perceived barriers to action and quality of life. Stepwise multiple regression showed that the most powerful predictor of quality of life was health-promoting behavior. The combination of health-promoting behavior, activity-related affect, self-esteem, health perception, commitment to planned action, social support, and perceived barriers to action accounted for 57% of the variance in quality of life. Conclusions: Multiple social, psychological, and health-related factors were associated with quality of life.