Pulmonary mechanics were evaluated in 13 nonsmoking adults with acute, uncomplicated influenza A/Port Chalmers/73(H3N2) virus infection. Subjects had no evidence of lower respiratory tract involvement on physical examination. Viral and/or scrologic evidence of influenza infection was established for all subjects. Physiologic measurements included forced expiratory rates by spirometry and total pulmonary resistance (RT) measured at 3, 5, 7, and 9 cycles per sec by the oscillometric technique. Subjects were studied at time of acute illness and at 1, 3, and 5 weeks thereafter. Spirometric measurements were initially normal in all subjects and did not change significantly throughout the study. Elevated RT measured at 3 cycles per sec and an abnormal degree of frequency dependence of RT, determined by comparing RT at 3 cycles per sec to RT at 9 cycles per sec, were initially present in 10 of 13 subjects. All 7 of these subjects initially abnormal who were tested 7 days after the onset of illness had abnormal frequency dependence. At 3 weeks, 7 of the 10 subjects initially abnormal continued to have frequency dependence of RT. All but 2 subjects reverted to normal levels by 5 weeks. Frequency dependence of RT indicates that acute influenza infection produces uneven time constants in the airways. A generalized increase in peripheral airway resistance or localized compliance changes could account for these abnormalities. Because these abnormalities persisted well beyond the period of clinical illness, these data imply that asymptomatic mechanical dysfunction of the lungs is a frequent sequela to acute influenza A virus infection. RT measured by the oscillometric method was a suitable noninvasive method for detecting abnormal airway time constants in these subjects with acute upper respiratory diseases.