According to Bandura, a person's expectations that a favorable outcome will follow a particular behavior are not sufficient to promote the occurrence of the behavior; the person must also believe that he or she will be effective at performing the behavior. The latter is referred to as self-efficacy. It has become a major focus in assessing patient performance of skills required to manage their illness. The present paper describes the development, testing, and applicability of an instrument for assessing self-efficacy in asthmatic patients. It notes that the Asthma Self-Efficacy Scale is not only a reliable paper-and-pencil instrument, but that it has a wide potential applicability throughout health care settings in measuring self-efficacy in asthmatic patients. The Asthma Self-Efficacy Scale is included.