Since HIV-2 was isolated in 1986, only 1 case of acute HIV-2 infection has been reported. We have identified another patient with primary HIV-2 infection. Follow-up samples were requested from the patient due to discrepant results. The HIV-2 infection was confirmed with HIV-2-specific proviral DNA amplification by PCR. The HIV-2 seroconversion panel obtained was used to evaluate the sensitivity of both combined and specific ELISAs currently in use in Europe, and to investigate the Western-blot patterns on both HIV-1-and HIV-2-specific Western blots. The window period was determined to be less than 37 days with the most sensitive assays. A remarkable difference in sensitivity to HIV-2 antibodies in acute HIV-2 infection was found in combined HIV-1/HIV-2 ELISAs. Three out of the 4 combined sandwich ELISAs appeared to be less sensitive than the indirect ELISAs in HIV-2 seroconversion, leading to a prolonged window period. One HIV-2-specific ELISA was also negative on the first sample, but positive on the second sample. In the HIV-2 Western blot, early reaction with HIV-2-specific env and gag proteins was seen, whereas the HIV-1 Western blot on the first sample revealed gag (p24, p55) reactivity only.