It has been suggested that during tidal breathing, the time to maximal expiratory flow, as a proportion of total expiratory time (Tme/TE), can be used as an index of airways obstruction. However, the relationship of Tme/TE to lung mechanics in infants is unclear. We examined this relationship in 42 premature infants (21 intubated and 21 not intubated) by comparing direct measures of pulmonary mechanics (lung resistance, dynamic lung compliance, and lung impedance) with Tme/TE measured over the same sample of tidal breathing. Tme/TE was positively correlated with lung compliance, expressed as a percent of the predicted value, in both intubated (r = 0.69, p < 0.005) and nonintubated (r = 0.64, p < 0.02) infants. There was no significant association between Tme/TE and lung resistance, expressed as a percent of the predicted value, in intubated (r = 0.32) and nonintubated (r = 0.23) infants. Tme/TE also showed, in nonintubated infants, a positive association with lung impedance, expressed as a percent of the predicted value, on the basis of the predictive values for compliance and resistance at the infant's unique respiratory rate, but this was significant only because of the influence of compliance on Tme/TE. These findings suggest that, in infants, the relationship between pulmonary mechanics and Tme/TE is complex, with Tme/TE being influenced by the elastic rather than the flow-resistive properties of the lungs.