Despite the fact that many individuals express a desire for more information and involvement in the health care process, it remains to be seen if they have adopted a more participative approach by becoming involved in decisions made about their health. Research indicates that, in actual practice, individuals are differentially willing or able to be active patients. AIDS patient activists are 1 group of individuals who have become more involved in their health care decision making. This study tests the reliability and validity of a measure of patient activism-the Patient Self-Advocacy Scale (PSAS)--designed to assess the dimensions of (a) increased illness and treatment education, (b) increased assertiveness in health care interactions, and (c) increased potential for nonadherence. Tests administered to 2 samples of participants (174 adults from an HIV-AIDS population and 21 8 adults from a general population) demonstrated that the PSAS was a reliable and valid measure of patient involvement in health care decision making.