We studied maximal expiratory flows at functional residual capacity (VmaxFRC) obtained by use of the chest compression technique in 9 infants who had signs of upper respiratory illness (URI) at the time of testing, and in 9 infants who were symptom-free but whose parents reported they had a URI in the previous month. When compared to 109 infants with no URI, infants with current URI had 40% lower VmaxFRC (mean +/- SD: 125.7 +/- 54.5 mL/s vs. 73.6 +/- 53.6 mL/s; P less than 0.01). Infants with a past URI had mean values for VmaxFRC (120.2 +/- 50.2 mL/s) that were not significantly different from those of infants with no URI. Changes in the shape of the flow-volume loop analogous to those reported in infants with lower airway obstruction were also noticed in infants with current URI. These findings suggest that, as in older children and adults, clinically unapparent alterations in lower airway function occur during URI in infants.