Human infections with avian influenza viruses can be severe and may be harbingers of the evolution of a pandemic strain. We present a patient in Thailand who was infected with influenza A (H5N1) virus. Prominent features included the progression from fever and dyspnea to the acute respiratory distress syndrome in a short period, lymphopenia and thrombocytopenia. Establishing the diagnosis for this patient increased public awareness of the virus and was soon followed by a halting of poultry-to-human transmission. On the basis of available data, any child with suspected avian influenza infection should be treated with oseltamivir.