Purpose: The purpose of this study was to identify the assertive behavior of asking smokers not to smoke and investigate the factors related to assertive behavior in patients with vascular diseases. Methods: Participants were 203 adult Korean patients with vascular diseases such as cerebral infarction and myocardial infarction. Data were collected using questionnaires that included the characteristics of secondhand smoke (SHS), secondhand smoke-related variables (Health belief model factors, health promotion model factors) and level of assertive behavior. Descriptive statistics, t-test, ANOVA and multiple regression using SPSS/WIN 18.0 were performed. Results: Participants who never ask smokers not to smoke was 39.9%, whereas participants who always ask was 7.4%. There was a weak positive relationship between assertive behavior and susceptibility to disease (r=.18), severity of disease (r=.19), benefit of assertive behavior to SHS exposure (r=.10), barrier of assertive behavior to SHS exposure (r=.24), and self-rated health (r=.21) respectively. There was a moderate positive relationship between assertive behavior and self-efficacy of assertive behavior to SHS exposure (r=.49). Health belief model factors explained 15.7% variance and health promotion model factors explained 27.0% of assertive behavior. Conclusion: The findings of this study suggest that self-efficacy of assertive behavior to SHS exposure is a very important factor. Therefore the development of a program to foster self-efficacy of assertive behavior regarding SHS exposure in patients with vascular diseases is needed.