To compare low birth weight (LBW:<2,500 g) between infants born to adolescent and adult mothers in Iquitos, Peru. A random sample of 4,467 records of women who delivered at the Hospital Apoyo Iquitos between 2005 and 2007 was collected from hospital birth registries. Multivariate logistic and linear regression analyses were performed to compare LBW in newborns of adolescents (10-14, 15-19 years) and adults (≤20 years) and then for primiparous mothers with a normal gestational age, adjusting for newborn sex, antenatal care, and location of the mother's residence. A total of 4,384 mothers had had a singleton live birth and 1,501 were primiparouswith a normal gestational age. Early and late adolescents had significantly greater odds of having a LBW infant than adults (OR = 2.28, 95%CI: 1.09, 4.78; OR = 1.67, 95%CI: 1.30, 2.14, respectively). For primiparous motherswith a normal gestational age, the same was true only for early adolescents (OR = 3.07, 95%CI: 1.09, 8.61). There were significant differences in mean birth weight between adults (3178.7 g) and both adolescent age groups overall (10-14 years: 2848.9 g; 15-19 years: 2998.3 g) and for primiparous mothers with a normal gestational age (10-14 years: 2900.8 g; 15-19 years: 3059.2 g; ≤20 years: 3151.8 g). Results suggest there is an important difference between adolescent and adult mothers in terms of newborn birth weight, especially among early adolescents. Future research on LBW and possibly other adverse birth outcomes should consider early adolescents as a separate sub-group of higher risk.