We conducted 151 tests of pulmonary function (PF) on 72 healthy infants younger than 2 yr of age using partial expiratory flow volume (PEFV) maneuvers and helium dilution determination of FRC. After tests were grouped into four strata based on postconception (PC) age, variability and sex differences in level of PF were examined. No significant sex differences were found for any PF measure in any age stratum, even when somatic size was controlled by length correction. Force expiratory flow measures, however, tended to be greater in girls than in boys in the youngest infants. Flow measures demonstrated greater between-subject variability than did volumes, and variability was greatest in the youngest infants. Within-subject variability also was more pronounced for flow measures, particularly in infants younger than 50 PC wk of age. Across the age range of infants studied, all PF measures were related linearly to somatic size as measured by either length, weight, or chest circumference. Length offered the best individual size correction of the three size parameters studied. Linear regression of PF parameters versus length demonstrated FRC to increase at 5.39 ml/cm over this age range, whereas flow at FRC increased by 9.67 ml/s/cm. We conclude that the variability of infant PF measures is greatest in early infancy, that measures of forced expiratory flow are more variable than volume measures, and that sex differences in infant PF do not appear significant. Length is related linearly to PF measures and offers reasonable size correction for healthy infants younger than 18 months of age.