The implications of recent weight gain trends for widening social disparities in body weight in the United States are unclear. Using an intersectional approach to studying inequality, and the longitudinal and nationally representative American's Changing Lives study (1986-2001/2002), we examine social disparities in body mass index trajectories during a time of rapid weight gain in the United States. Results reveal complex interactive effects of gender, race, socioeconomic position and age, and provide evidence for increasing social disparities, particularly among younger adults. Most notably, among individuals who aged from 25-39 to 45-54 during the study interval, low-educated and low-income black women experienced the greatest increase in BMI, while high-educated and high-income white men experienced the least BMI growth. These new findings highlight the importance of investigating changing disparities in weight intersectionally, using multiple dimensions of inequality as well as age, and also presage increasing BMI disparities in the U.S. adult population.