Context: Early life cortisol plays an important role in bone, muscle, and fat mobilization processes, which could influence body composition, affecting anthropometric indicators such as weight and height. Objective: To explore the association between diurnal cortisol levels and growth indexes in children from 12 to 48 months of age. Design: This study includes data from 404 children from the Programming Research in Obesity, Growth, Environment and Social Stressors Mexican birth cohort. Cortisol was measured in eight saliva samples collected at four time points during the day (from wakeup to bedtime), over 2 days, when the child was either 12, 18, or 24 months old. Total daytime cortisol levels were calculated by averaging the area under the curve (AUC) for the 2 days. Height and weight were measured from 12 to 48 months of age. Growth indexes were constructed according to z scores following World Health Organization standards: weight-for-age z score (Z-WFA), height/length-for-age z score, weight-for-height/length z score (Z-WFH), and body mass indexfor-age z score (Z-BMIFA). Mixed models were used to analyze the association between cortisol AUC quartiles and growth indexes. Results: Cortisol showed an inverted U-shaped association with the four growth indexes. Compared with the first quartile, all quartiles had a positive association with indexes that include weight, with the second quartile having the strongest association, resulting in an average change of b (95% CI) 0.38 (0.130.64) for Z-WFA, 0.36 (0.100.62) for Z-WFH, and 0.43 (0.170.69) for Z-BMIFA. Conclusions: Results suggest that early life daytime cortisol levels, as a reflection of hypothalamicpituitary-adrenal axis development, might influence growth in early infancy.