The present study was designed to assess the influence of breathing pattern on the variations of functional residual capacity during sleep in newborn infants. Functional residual capacity was measured by the He-dilution method. Neurophysiologic criteria were used to identify sleep states. Movements of chest and abdomen were monitored. Twenty-six healthy newborn infants were studied. Sixteen were premature and 10 were at term. Functional residual capacity did not change in relation to changes in sleep states. In active sleep it was 1.48 +/- 0.07 ml/cm compared with 1.50 +/- 0.06 ml/cm in quiet sleep. Functional residual capacity decreased when rib cage and abdomen moved out-of-phase with a value of 1.38 +/- 0.09 ml/cm as compared to 1.56 +/- 0.09 ml/cm when in phase (P less than 0.01), in the 7 infants who displayed these two opposite patterns.