In this study we have examined the feasibility of a program based on the health belief model (HBM), for its effectiveness in improving lifestyle-related diseases in patients with schizophrenia (SZ) and bipolar disorder (BD), which are often complicated with physical conditions. In this model, we attempted to enable patients to identify a “threat” and to find “balance between benefits and disadvantages”. Subjects were carefully selected from among psychiatric patients by excluding any bias. Thus, the enrolled patients were 30 adult men and women with lifestyle-related diseases, or those with a body mass index (BMI) of over 24. Of these 30 subjects, 15 were randomly assigned to the intervention group and 10 the control group, since 5 subjects in the control voluntarily left from the study. Comparison of the intervention and control groups revealed significant improvement (p< 0.05) in HDL cholesterol in the intervention group. However, there were no significant changes in other variables. These findings support the usefulness and efficacy of HMB-based nutritional interventions for preventing lifestyle-related disorders among psychiatric patients. Further evaluation is needed with a larger sample size and a longer intervention period. This HMB-based intervention could be useful for the general population as well.