The recent appearance and worldwide spread of novel influenza A (H1N1) virus has highlighted the need to evaluate commercially available, widely used, rapid influenza diagnostic tests (RIDTs) for their ability to detect these viral antigens in respiratory clinical specimens. As an initial assessment, CDC conducted an evaluation of multiple RIDTs. Sixty-five clinical respiratory specimens collected during April-May 2009 that had previously tested positive either for novel influenza A (H1N1) or for seasonal influenza A (H1N1) or A (H3N2) viruses by real-time reverse transcription--polymerase chain reaction (rRT-PCR) assay were used in the evaluation. The results showed that, although the RIDTs were capable of detecting novel A (H1N1) virus from respiratory specimens containing high levels of virus (as indicated by low cycle threshold [Ct] values), the overall sensitivity was low (40%-69%) among all specimens tested and declined substantially as virus levels decreased (and Ct values increased). These findings indicate that, although a positive RIDT result can be used in making treatment decisions, a negative result does not rule out infection with novel influenza A (H1N1) virus. Patients with illnesses compatible with novel influenza A (H1N1) virus infection but with negative RIDT results should be treated empirically based on the level of clinical suspicion, underlying medical conditions, severity of illness, and risk for complications. If a more definitive determination of infection with influenza virus is required, testing with rRT-PCR or virus isolation should be performed. Additional evaluations of the accuracy of RIDTs in detecting novel influenza A (H1N1) virus should be conducted.