The objective of this study was to test three hypotheses concerning age-related differences in the composition of the weight gained by pregnant adolescents. We studied the change in maternal body composition during gestation in relation to maternal age and infant birth weight in relation to change in maternal body composition in 108 consecutive, low-income, pregnant, black adolescents. Regardless of age, approximately 26% of adolescents' body weight and 38% of the weight adolescents gain during gestation is fat. A hierarchical regression analysis revealed that the rate of gestational weight gain is significant predictor of infant birth weight among younger (R2 change = 0.15, P = 0.004), but not older (R2 change = 0.01, not significant), adolescents. Among younger adolescents, the rate of fat weight gain is the best predictor of infant birth weight. Maternal age interacts with fat weight gain (but not total or lean weight gain) to predict infant birth weight (R2 change = 0.046, P = 0.022). During gestation maternal age does not have a clinically important effect on the composition of the body or weight gained by pregnant adolescents. However, maternal age interacts with the composition of the gestational weight gain to predict infant birth weight; the fat component of the weight gain is a significant predictor of infant birth weight in younger, but not older, adolescents.