With a parallel increase in the consumption of food away from home, particularly fast food, and the obesity prevalence in the United States, evidence on the potential effectiveness of fiscal pricing policies to curb obesity is needed. We estimate changes in the dispersion of the entire conditional distribution of body mass index (BMI) associated with changes in fast food prices for adults using the National Longitudinal Survey of Youth 1979 in cross‐sectional and longitudinal quantile regression models. We find that the ordinary least squares estimate for men underestimates the negative relationship of fast food prices with BMI at the 50th and upper quantiles in cross‐sectional models although the statistical significance disappears in the longitudinal individual fixed effects quantile regression. Among subpopulations, we find that a 10% increase in the price of fast food is associated with 0.9% and 0.7% lower BMI for low‐income women and women with any children, respectively, at the 90th quantile in a longitudinal individual fixed effects model. Our results imply that fiscal pricing policies such as fast food taxes might have a greater impact on the weight outcomes of low‐income women or women with children in the upper tail of the conditional BMI distribution (JEL I00, I19).