Between October 30 and November 5, 1980, the Professional Association of Interns and Residents of Ontario called a strike of house staff in Ontario's teaching hospitals. The authors obtained data concerning utilization of laboratory tests and radiology procedures during that period and for the same days 2 weeks before and after the strike. During the strike period, the number of tests performed per patient day decreased by only 8.3%. After accounting for proportional changes in emergency and nonemergency admissions, there was no significant change in the number of tests or relative value units performed per patient day as a result of the strike. These results suggest that the volume of tests performed in teaching hospitals is more likely related to the case mix and severity of illness of patients admitted to these institutions than to a pure “teaching effect.”