Excess weight is becoming widespread in Spain due to changes in nutritional habits and lifestyles. Previous studies on this issue have focused on specific Spanish regions, subpopulations or relatively short time spans. This study analysed sex, age and cohort trends in the prevalence of adult overweight and obesity over the last two decades by applying a demographic methodology. Data came from the Spanish National Health Surveys that were held between 1987 and 2006. The respondent's demographic characteristics and self-reported height and weight were aggregated to a single dataset in order to analyse changes in weight and BMI by age and sex, over time and within and between quasi birth-cohorts. After correcting for sample bias and coding errors a total sample of about 100,000 subjects aged 20-79 was obtained. The results show that between 1987 and 2006 adult males and females increased their average weight by 8.2% and 2.8%, respectively. While among younger adults this is partly explained by height increases, prevalence in excess weight increased among 50- to 79-year-old males. Persons of the same 10-year age group but of a more recent 10-year quasi birth-cohort had a BMI that was 0.2-0.8 points higher. BMI increases were lower for women and mainly affected 60-79 year olds. In fact, even decreases were observed for 40- to 49 and 50- to 59-year-old women. Potential explanatory factors are discussed.