Using the dualistic model of passion (Vallerand et al., 2003), this research investigated how harmonious passion (HP) or obsessive passion (OP) for a cause can affect volunteers' health and subjective well‐being. Three studies with volunteers for local (local emergency crises and community help) and international (humanitarian missions) causes assessed physical and psychological health using cross‐sectional and longitudinal designs. Study 1 (N = 108) showed that HP was positively related to satisfaction with one's involvement in the cause and unrelated to physical injuries due to cause involvement. OP was unrelated to satisfaction but positively associated with injuries. Findings were replicated in Study 2 (N = 83). Moreover, self‐neglect mediated the positive and negative effects of HP and OP, respectively, on injuries. Study 3 (N = 77) revealed that HP predicted an increase in satisfaction and health over a 3‐month mission. OP predicted an increase in physical symptoms and a decrease in health. Furthermore, OP before a mission was positively related to self‐neglect that was positively associated with physical symptoms after a mission. OP also positively predicted rumination that was conducive to posttraumatic stress disorder. HP was unrelated to these variables. Findings underscore the role of passion for a cause in predicting intrapersonal outcomes of volunteers.