The prevalence of obesity in Finnish children, adolescents and young adults aged three to 24 years was estimated in three surveys performed within the multicentre project, "Cardiovascular Risk in Young Finns" (1980, 1983, 1986). Obesity was defined as either body mass index (weight/height) or skinfold thickness (triceps or subscapular) or both greater than 90th percentiles of age and sex-specific reference data for white children. Its mean prevalences among 9- to 18-year old boys and girls in three surveys (95% confidence limits) were 3.6% (3.1-4.2) and 2.1% (1.7-2.6) as estimated in terms of body mass index and triceps skinfold thickness or 4.3% (3.9-4.9) and 2.6% (2.2-3.1) according to body mass index and subscapular skinfold thickness. Thus the 9- to 18-year old boys were on average more often obese than the girls, but no statistically significant changes in the prevalence of obesity were observed over the period 1980-1986. Body mass index and triceps or subscapular skinfold thicknesses vary in sensitivity as indicators of obesity.