Valid and reliable assessment of the clinical decision-making process is essential for the evaluation of decision aiding methods and effective quality assurance programs. The Provider Decision Process Assessment Instrument is a 12-item questionnaire that measures a health care provider's degree of comfort with a medical decision. Its measurement properties were studied in two general internal medicine practices. Reliability, measured using Cronbach's alpha, was 0.90 (95% Cl = 0.87 to 0.92). Construct validity was also high, with expected negative correlations ranging from -0.53 to -0.67. The instrument also satisfied standard criteria for item homogeneity and was readily completed by clinicians. These results suggest that the Provider Decision Process Assessment Instrument will prove to be a valuable tool for assessing medical decision making in busy clinical settings.