OBJECTIVE. Previous studies have suggested that the timing of puberty is associated with BMI in childhood and adult stature. Because the genetic background of these associations is not thoroughly investigated, we aimed to analyze it in a longitudinal twin cohort. METHODS. We studied a Swedish cohort of 99 monozygotic and 76 dizygotic twin pairs born between 1973 and 1979 with weight and length or height measured annually from birth to age 18 years. Age at onset of pubertal growth spurt, age at peak height velocity, and final height were estimated by a parametric JPA2 growth model. The genetic architecture and mutual associations of these traits and childhood BMI were analyzed by linear structural equation modeling. RESULTS. The heritability estimate was 0.91 for age at onset of pubertal growth spurt, 0.93 for age at peak height velocity, and 0.97 for adult height. Age at onset of pubertal growth spurt was negatively associated with BMI from 1 to 10 years of age and stature in early adulthood. For age at peak height velocity, we found similar associations with childhood BMI and stature in early adulthood. These associations were explained by common genetic factors. CONCLUSION. Growth during puberty is strictly genetically regulated. These genetic factors also explain why boys who matured early had higher BMI through childhood and taller stature in early adulthood.