An optical immunoassay for the rapid detection of influenza types A and B viral antigens, FLU OIA (Biostar, USA), was prospectively compared with antigen detection methods and cell culture on 400 respiratory specimens during an influenza outbreak that occurred in Switzerland in 1998/1999. The FLU OIA had an overall sensitivity of 64.4% (95%CI, 56.3-71.7%) and a specificity of 94.9% (95%CI, 89.8-97.7%). Using specimens from pediatric and adolescent patients, the sensitivity obtained (71.8%; 95%CI, 61.7-80%) was different than that achieved with specimens from adult patients (51.4%; 95%CI, 36.5-65%) (P=0.004). The results show that rapid diagnostic tests with higher sensitivity and specificity for the detection of influenza virus are needed.