Influenza virus A (H5N1) was isolated from the tracheal swab of a 3-year-old boy who died from influenza with the Raye syndrome in Hong Kong in May, 1997. Up to the present time, influenza viruses with hemagglutinin H5 were known to circulate only among birds. They caused a variety of diseases: from asymptomatic to epizootic with 100% mortality, particularly among chickens. The main difference between virulent and avirulent strains is as follows: virulent viruses are isolated from all tissues of an infected bird. A (H5) virus hemagglutinin, transformed into a virulent variant, becomes sensitive to cleavage by proteases of mammalian and avian cells. Intensive epidemiological surveillance of influenza in Hong Kong started by the WHO and Department of Public Health of Hong Kong in August-September, 1997, resulted in detection of 17 more cases with Influenza A (H5N1) in November-December 1997. all of the occurred before December 28, 1997 and were detected in hospitals and health centers of Hong Kong. Nine patients were children aged under 5 years. Six patients died as a result of complications (pneumonia) and exacerbations of concomitant chronic diseases. Virological and logical studies showed that the main route of infection transmission was from birds to humans. Human to human transmission is probable. Study of 7 influenza A (H5N1) viruses isolated from patients showed that they contained all 8 RNA gene segments of avian virus. There are no reports about new cases of influenza A (H5N1) in humans in January 1998, and we can hope that the outbreak of Influenza A (H5N1) in Hong Kong caused by avian virus will not develop into a new influenza pandemic, although an unfavorable course of events is probable.