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OBJECTIVE: To study the trends in overweight and obesity among Finnish adolescents in 1977-1999. DESIGN: Mailed surveys every other year. SUBJECTS: Nationally representative samples of 12, 14, 16 and 18-y-olds (n = 64 147, response rate 78.9%). METHODS: Overweight and obesity were measured by body mass index (BMI) and relative weight (RW) based on self-reported height and weight. BMI greater than or equal to the 85th percentile cut-off point for BMI in each age- and sex-specific group in the entire data set was considered as overweight, and BMI greater than or equal to 95th percentile cut-off point as obesity. RW greater than or equal to 110% and greater than or equal to 120%, calculated as the individual's weight divided by the mean weight in each age- and sex-specific height percentile group in the entire data set, were considered as overweight and obesity, respectively. The trends in overweight and obesity are described by the change in the 85th and 95th percentile cut-off points of BMI over time. The prevalence of overweight and obesity is also reported using BMI reference values recommended for international comparisons. Because of the similarity of the BMI and the RW criteria in classifying adolescents as overweight and obese, only results based on BMI are presented. RESULTS: Overweight and obesity increased linearly in all sex and age groups from 1977 to 1999. Depending on the age group, the average increase in the 85th percentile cut-off point of the BMI per 10y was 0.6-1.1 kg/m(2) in boys and 0.3-0.7 kg/m(2) in girls. The 95th percentile cut-off point of the BMI for boys and girls increased by 1.1-1.6 kg/m(2) and by 0.6-1.0 kg/m(2) per 10 y, respectively. In boys, the increase in overweight and obesity was largest in the two youngest age groups. In girls, the increase in overweight was largest in the oldest age group, and that of obesity both in the 14 and 18-y-olds. Overweight and obesity increased more in boys than in girls in all age groups except in the 18-y-olds among whom the increase was similar in both sexes. Examination of the entire BMI distribution showed that there was little or no change over time at the lower (5th, 15th) and middle (50th) percentiles, but increasing differences at the upper end of the distribution, the increases in the 95th percentile being even more marked than those in the 85th percentile curves. According to international reference values, the age-standardized prevalence of overweight increased in boys from 7.2 to 16.7%, and in girls from 4.0 to 9.8%, between 1977 and 1999. The prevalence of obesity in boys was 1.1% in 1977 and 2.7% in 1999, and in girls 0.4 and 1.4%, respectively. CONCLUSION: Overweight and obesity increased remarkably among Finnish adolescents from 1977 to 1999. The changes concentrated at the upper end of the BMI distribution, suggesting that factors behind this development have influenced only a part of the adolescent population.