Weight-for-length during the early postnatal period is a critical predictor of subsequent body composition and metabolic risk. This study was designed to analyze change in weight-for-length status according to birth weight in early infancy. Data were collected for 267 infants enrolled in the Jackson County Women, Infants, and Children (WIC) program. Postnatal measurements were collected at a clinic visit between birth and 12 weeks of age (mean = 5.7 weeks). Changes inWHO z-scores (weight, length, weight-for-length) between birth and the clinic visit were calculated. Infants were classified as exclusively breastfed or as formula-fed. Ethnicity was coded as Hispanic or non-Hispanic. Infants were classified based on birth weight z-score as lower (<−1 SD) or higher (> +1 SD). Multiple regression models tested birth weight, demographic factors, and feeding as predictors of z-score change measures. Demographic factors and feeding were also tested as moderators of the effects of birth weight. Lower birth weight infants displayed an increase in weight-for-length z-score between birth and the clinic visit. Change in weight-for-length was associated with significant increase in weight z-score but not in length z-score. Higher birth weight predicted decrease in weight and length z-scores but did not predict change in weight-for-length. Hispanic ethnicity predicted decrease in length z-score and increase in weight-for-length z-score but did not moderate effects of birth weight. Increase in weight-for-length among lower birth weight infants and persistence of high weight-for-length among higher birth weight infants may reflect phenotypic adjustments that are maladaptive in adverse dietary environments. Am J Phys Anthropol, 2013. © 2012 Wiley Periodicals, Inc.