To examine the relationship between high (≥5%) weight loss during the first 24 h after birth and eventual excess weight loss (EWL) of ≥10% of birth weight. Retrospective cohort study. Kaiser Permanente Northern California hospitals. 63 096 infants born at ≥36 weeks in 2009-2010, of whom 59 761 (94.5%) had a weight subsequent to birth weight measured at <24 h. Per cent of birth weight lost by 24 h of age. Weight nadir, defined as the lowest recorded inpatient or outpatient weight in the first 30 days after birth, expressed as a percentage of birth weight. Among infants who breastfed at least once, mean (±SD) weight nadir was 6.3±3.5% below birth weight, and 9.6% of the newborns lost ≥10% of birth weight. Among 2670 infants who lost ≥5% of their birth weight in the first 24 h, 782 (29%) eventually developed EWL, compared with 4840 (8%) of 57 109 infants who lost <5% (p<0.0005). In multivariate analysis, ≥5% first-day weight loss predicted eventual EWL (≥10%) with an OR of 4.06 (95% CI 3.69 to 4.46) after adjusting for gestational age, method of delivery, maternal race/ethnicity and hospital of birth. High first-day weight loss predicts eventual weight nadir and can be used to identify infants who might benefit from targeted interventions to support breastfeeding and prevent EWL.