The theory of planned behavior (TPB) has been successfully applied to various health behaviors. Previous research using this theory has not described the health behaviors of Taiwanese people diagnosed with various different chronic illnesses. Because TPB constructs and determinants are abstract, qualitative interviews were conducted with 36 people with various chronic illness diagnoses to identify such determinants and establish measurement indicators for further quantitative study. Content analysis of responses identified the determinants of attitude affecting adoption of health behaviors, both in terms of encouraging and discouraging adoption. The major referents that influenced health behavior adoption included family members, social networks, and health professionals. The determinants of perceived behavioral control included making health behavior easy and hindering health behavior. These findings provide a new glimpse into the application of the theory of planned behavior on the health behavior of the chronically ill population.