Regular surveillance of influenza virus activity and associated illness is necessary to monitor changes in circulating strains. As part of a demonstration project, medical practices in a seven-county area in southern lower Michigan were recruited as sentinel surveillance sites. The practices were asked to collect specimens for virus isolation and/or data on age-specific frequency of respiratory illnesses that met a case definition. Data were used to establish periods of influenza virus activity for vaccine effectiveness determinations. Over three study periods, 1989-1992, there were documented outbreaks of influenza type A(H3N2), type A(H1N1), and type B. The incidence of influenza-like illness increased during these outbreaks, with variation in age-specific frequency from year to year. Recovery of virus was not affected during the winter season by long delays in specimen receipt by the laboratory. Results indicated that medical practices can be successfully recruited as surveillance sites to provide information on the timing, etiology, and intensity of acute respiratory illness.