This study examined the reliability and validity of a brief six-item instrument to measure the mental workload experienced by residents for specific patient visits to an ambulatory care clinic. Participating in the study were twenty-two residents in postgraduate years 1 through 3 who were working in the general outpatient clinic of an inner city, private, nonprofit community hospital. Cronbach's alpha coefficient for the instrument was 0.80. Findings supported several theory-based hypotheses on determinants and performance consequences of mental workload. Mental workload was positively correlated with fatigue (r = 0.42, P = 0.03) and inversely correlated with residents' self-rated experience with patients' problems (r = -0.65, P less than 0.001). Residents' performance was measured through self-ratings and faculty physicians' ratings. Mental workload was inversely correlated with self-rated performance (r = -0.67, P less than 0.001). The correlation of mental workload with faculty physician ratings that reflected the technical dimension of patient care (physician examination, medications, and procedures) was r = -0.38 (P = 0.04). With mental workload squared, the correlation was r = -0.45 (P = 0.02) and the form of the relationship, consistent with the hypothesis, was a slightly downward sloping curve. Limitations of this research are discussed as well as suggestions for further research.