Objective To determine whether birth weight is associated with handgrip strength and cardiovascular fitness if) adolescence and, if so. how these associations are influenced by current body composition. Study design A total of 1801 adolescents (983 females). age 13 to 18.5 years, from the AVENA (Alimentacion y Valoracion del Estado Nutricional de los Adolescentes Espanoles [Food and Assessment of the Nutritional Status of Spanish Adolescents]) study were evaluated. Handgrip strength and cardiovascular fitness were assessed using the handgrip test and the 20-m shuttle run test. respectively. Results Birth weight was positively associated with handgrip strength in females after controlling for current age, gestational age, breast-feeding. and adolescent body mass index (P = .002), body fat percentage (P < .001), or,waist circumference (P = .005), but not after controlling for fat-free mass. The associations were similar yet weaker in males. Females with high birth weight (>90th percentile) had greater handgrip strength than those with normal (10th to 90th percentile) or low (<10th percentile) birth weight, after adjusting for body fat percentage (P = .004). All of the differences became nonsignificant after adjusting for adolescent fat-free mass. Birth weight was not associated with cardiovascular fitness. Conclusions High birth weight is associated with greater handgrip strength in adolescents. especially in females. vet these associations seem to be highly explained by fat-free mass. (J Pedicar 2009:154:61-6)