Global respiratory muscle strength was studied in 22 normal healthy volunteers during a 4-month winter period. Twelve subjects developed naturally acquired upper respiratory tract infections. Maximal static expiratory and inspiratory mouth pressures fell significantly during these infections. The greatest falls were documented between the third and seventh days of clinical illness. Full recovery occurred by the fourteenth day. We conclude that significant abnormalities of respiratory muscle function can occur during upper respiratory tract infections in otherwise healthy young adults.