When not available to their patients, family practitioners in large cities can "sign out" to deputizing agencies, which coordinate the activities of part-time physicians on call. The physicians making use of one such agency in Toronto appeared to be representative of family practitioners in the region. One thousand of their patients seen consecutively by one physician were asked a series of questions. The majority of the 811 evaluable patients were considered to have problems justifying an after-hours call, although 16% of the problems were of a trivial nature. However, almost half of the patients with trivial problems would have gone to the local emergency room, as would 85% of all the patients. Although virtually all (94%) of the visits with the elderly were justified, 34% of the patients in this age group said they probably or definitely would not have sought emergency room care if a physician had not been available. The use of deputizing agencies should reduce the overuse of emergency room facilities and provide optimal after-hours primary medical care.