A reliable emergency department (ED) workload measurement tool would provide a method of quantifying clinical productivity for performance evaluation and physician incentive programs; it would enable health administrators to measure ED outputs; and it could provide the basis for an equitable formula to estimate ED physician staffing requirements. Objective: Our objectives were to identify predictors that correlate with physician time needed to treat patients. Methods: During 30 days, evening, night and weekend shifts, researcher shadowed emergency physician, documenting time spent performing clinical and non-clinical functions for 585 patient visits. The recorded key predictors included patient gender, age, vital signs and Glasgow Coma Scale (GCS) score, and the mode of arrival, triage level assigned, comorbidity and procedures performed. Results: The strongest predictor variables were: procedure required, triage level, arrival by ambulance, GCS, age, any comorbidity, and number of prior visits. Conclusions: This study clarifies important determinants of emergency physician workload.