BACKGROUND AND OBJECTIVES: The underlying influences on different patterns of BMI development are not well understood, and psychosocial outcomes linked to BMI development have been little investigated. Objectives were to identify BMI developmental trajectories across the first decade of life, examine early life predictors of trajectory membership, and investigate whether being on a particular BMI trajectory is associated with markers of psychosocial well-being. METHODS: We used latent class analysis to derive BMI trajectories by using data collected at ages 3, 5, 7, and 11 years on 16 936 participants from the Millennium Cohort Study. Regression models were used to estimate predictors of BMI trajectory membership and their psychosocial correlates. RESULTS: Four trajectories were identified: 83.8% had an average "stable" nonoverweight BMI, 0.6% were in a "decreasing" group, 13.1% had "moderate increasing" BMIs, and 2.5% had "high increasing" BMIs. Predictors of "moderate" and "high" increasing group membership were smoking in pregnancy (odds ratios [ORs] = 1.17 and 1.97, respectively), maternal BMI (ORs = 1.10 and 1.14), skipping breakfast (ORs = 1.66 and 1.76), nonregular bedtimes (ORs = 1.22 and 1.55). Children in the "moderate" and "high" increasing groups had worse scores for emotional symptoms, peer problems, happiness, body satisfaction, and self-esteem, and those in the "high increasing" group were more likely to have tried alcohol and cigarettes. CONCLUSIONS: Several potentially modifiable early life factors including smoking in pregnancy, skipping breakfast, and bedtime routines were important predictors of BMI development in the overweight and obese range, and high BMI growth was linked to worse psychosocial well-being.