Background: The level of work intensity associated with patient encounters has implications for quality of care, patient safety, practice management, and reimbursement. The utility of available instruments for clinical work intensity assessment is unknown. Objective: We assessed, in the clinical setting, the performance of existing measures of work intensity that are valid for nonclinical contexts. Research design: A cross-sectional, multimeasure design involving work intensity assessments for the last patient encounter and for an entire half-day clinic session. Subjects: A convenience sample of 14 providers from the following 4 specialties: family medicine, general internal medicine, neurology, and surgery. Measures: Perceived clinical work intensity was measured by the following 3 instruments: National Aeronautic and Space Administration-Task Load Index, Subjective Workload Assessment Technique, and Multiple Resources Questionnaire; stress was measured by the Dundee Stress State Questionnaire. Convergent validity was assessed by correlation among the instruments. Results: For the last patient encounter, there was a moderate to high correlation between the work intensity instruments' scores (Pearson's r ranged from 0.41 to 0.73) and low to moderate correlation with the distress subscale of the Dundee Stress State Questionnaire (Pearson's r ranged from -0.11 to 0.46), reflecting their stress dimension. Provider personality was associated with reported levels of work intensity and stress. Similar results were obtained when the entire clinic session was the unit of reference. Conclusion: Existing measures of work intensity and stress appear to be valid for use in the clinical setting to generate evidence on perceived intensity and stress experienced by providers in the performance of medical services. © 2010 by Lippincott Williams & Wilkins.