STUDY OBJECTIVE To develop categories of behavior that define an applicant's aptitude for anesthesia, and to attempt to determine the relative importance of these behaviors to successful residency performance. DESIGN Prospective open study. SETTING Anesthesia residencies at three midwest university teaching hospitals. INTERVENTIONS Using a structured interview format known as the critical incident technique, faculty anesthesiologists were asked to describe examples of effective and ineffective behaviors observed among anesthesia residents during the twelve months prior to the interview. MEASUREMENTS AND MAIN RESULTS Interviews initially held with 34 anesthesiologists generated 172 incidents. These incidents formed the basis for a categorization analysis performed by two anesthesiologists. Six categories were developed: preparedness, interpersonal skills, response to teaching, data monitoring, technical skills, and emergency situations. Validation of these categories was confirmed with three subsequent interviews, in which 92 anesthesiologists generated 475 incidents. Most incidents were found to conform to the previously defined categories using a reallocation index with a range of 0.70 to 0.80. The category "technical skills" fell below the defined range. Over 60 percent of the incidents involved noncognitive personal attributes: preparedness, interpersonal skills, and response to teaching. CONCLUSION Effective behavior in six categories identifies an applicant's aptitude for anesthesia. Selection of residents may be enhanced by routinely assessing noncognitive characteristics.