We examined if the timing of introduction of solid foods was related to growth, intake, morbidity, activity or motor development among infants either breast fed or formula fed until > or = 12 months of age. Breast-fed infants given solids before 6 months of age (earlysol) consumed less breast milk at 6 and 9 months of age than those given solids > or = 6 months (latesol); thus total energy intake did not differ between groups. Z scores for weight, length and weight-for-length at 1-18 months did not differ between groups. Latesol infants gained less weight from 6 to 9 months but not during any other interval. Neither activity level nor morbidity differed between groups, but several developmental milestones occurred earlier in the earlysol versus the latesol group, probably due to reverse causation. Among formula-fed infants, timing of introduction of solid foods was not related to intake, growth, activity or morbidity. We conclude that solid foods given before 6 months of age generally replace the milk source among breast-fed but not formula-fed infants.