OBJECTIVE In light of the obesity epidemic, we aimed to characterize novel childhood adiposity trajectories from birth to age 14 years and to determine their relation to adolescent insulin resistance. RESEARCH DESIGN AND METHODS A total of 1,197 Australian children with cardiovascular/metabolic profiling at age 14 years were studied serially from birth to age 14 years. Semiparametric mixture modeling was applied to anthropometric data over eight time points to generate adiposity trajectories of z scores (weight-for-height and BMI). Fasting insulin and homeostasis model assessment of insulin resistance (HOMA-IR) were compared at age 14 years between adiposity trajectories. RESULTS Seven adiposity trajectories were identified. Three (two rising and one chronic high adiposity) trajectories comprised 32% of the population and were associated with significantly higher fasting insulin and HOMA-IR compared with a reference trajectory group (with longitudinal adiposity z scores of approximately zero). There was a significant sex by trajectory group interaction (P < 0.001). Girls within a rising trajectory from low to moderate adiposity did not show increased insulin resistance. Maternal obesity, excessive weight gain during pregnancy, and gestational diabetes were more prevalent in the chronic high adiposity trajectory. CONCLUSIONS A range of childhood adiposity trajectories exist. The greatest insulin resistance at age 14 years is seen in those with increasing trajectories regardless of birth weight and in high birth weight infants whose adiposity remains high. Public health professionals should urgently target both excessive weight gain in early childhood across all birth weights and maternal obesity and excessive weight gain during pregnancy.