Physician mental work load is an important variable intervening between work demands imposed on physicians and physician performance. A brief instrument was developed to measure the mental work load experienced during a clinic session in internal medicine ambulatory care hospital clinics. The instrument covered six dimensions of mental work load: performance, time load, mental effort, physical effort, psychologic stress, and difficulty. Cronbach's alpha reliability coefficient for the instrument was 0.83. The instrument exhibited construct validity. As hypothesized, mental work load was found to be positively associated with number of patients seen and with fatigue, and mental work load was inversely associated with physician satisfaction with the patient care they provided and with their self-rating of the quality of care they provided. The importance of measuring physician mental work load is discussed.