Human metapneumovirus is an emerging human respiratory pathogen first discovered in 2001. It clinically resembles respiratory syncytial virus, can cause both upper and lower tract disease, and has been associated with serious illness in the young, among the immunosuppressed, and in the chronically ill. Cough and congestion are frequently reported, and respiratory failure may occur. Initial infection occurs during early childhood, and repeated infections throughout life impart only transient immunity. Diagnosis is by reverse transcription-polymerase chain reaction or rising serologic titers. No commercial laboratory tests or treatments are available. Primary care physicians should maintain vigilance for outbreaks of newly discovered and emerging respiratory illnesses.