Background Brazil is among the 10 countries with the highest prevalence of overweight in the world. The aim was to analyse the changes in the dispersion of body mass index (BMI) in the Brazilian population from the 1970s to 2013 in specific population groups.

Methods Data were extracted from five national household surveys between 1974–1975 and 2013, including adults aged 20 to 64. We calculated SD, median, fifth and 95th percentiles of BMI for each sociodemographic category (sex, age, schooling) and survey year in order to explore whether changes in mean BMI are followed by changes in dispersion.

Results During the period the mean BMI ranged from 22.7 kg/m² to 26.6 kg/m², with a much higher variation in the 95th percentile (+6.1 kg/m²) when compared with the fifth percentile (+1.8 kg/m²). The within-group differences increased over time. The SD increased in all categories analysed and was higher among women, lower schooling groups and the oldest group. An increase of 1.0 kg/m² in the BMI mean was associated with an increase of 0.32 kg/m² in the SD, 0.45 kg/m² in the fifth percentile and 1.50 kg/m² in the 95th percentile of BMI.

Conclusions Population changes occurred in BMI are more complex than the simple increase of its average. Concomitant to the increase in BMI, there was an increase in the within-group differences, showing that growing inequalities are not driven solely by sociodemographic factors.