The incremental benefits and costs of a rapid influenza A virus infection diagnostic service were studied in nursing homes in Calgary, Canada, during a single influenza season. The service was used to test 159 patients with suspected infection in a group of "experimental" nursing homes and results were compared with those for a group of "control" homes. An equal number of cases of influenza were identified in each group. Twenty-eight patients (17.6%) had confirmed cases, and 63 patients (39.6%) had probable cases. A rapid viral test result was provided much faster for patients in the experimental homes (P=.005). Both groups had the same median attack rate for influenza A virus. In experimental homes, the duration of the outbreak was shorter (P=.03), and the cost of laboratory testing and the total cost (less the hospital cost) tended to be lower (P<.2). The rapid testing service also tended to lower the overall use of resources.