Airways obstruction has been demonstrated in acutely wheezing infants. The aim of the present study was to assess functional abnormalities as detected by measurement of total respiratory system resistance (Rrs) and functional residual capacity (FRC) in infants with a history of recurrent episodes of wheezing, while not acutely ill. In 30 such infants (mean age, 10 months; range, 4-17) and in 10 healthy infants (mean age, 6 months; range, 0-14) four Rrs measurements, performed with the forced pseudo-random noise (PRN) oscillation technique, and three FRC determinations, using the closed-circuit helium dilution technique, were averaged. A lower than predicted FRC was demonstrated in 20/30 (66%) patients. At 16 Hz, Rrs was significantly above predicted in 3/30 (10%) patients. Specific Rrs (Rrs x FRC) at 16 Hz was increased in 5/30 (17%) patients. In conclusion, the PRN oscillation technique combined with FRC measurement by helium dilution detects lung function abnormalities in a minority of wheezing infants during symptom-free intervals.