The purpose of the study was to investigate the time trend of questionnaire-assessed moderate to vigorous physical activity (MVPA) among 12-18-year-old Finnish boys and girls from 1979 to 2005. The MVPA was defined as "at least moderately breathtaking and sweating in leisure time physical activities more than 3 times week." Data were based on the Adolescent Health and Lifestyle Survey, which is a biannual, nation-wide survey on adolescent health and health-related lifestyle in Finland. The nationally representative samples were drawn from the Finnish Population Register Centre. The numbers of participants varied from 2832 to 8390 and the response rates from 91% (girls, 1981) to 58% (boys, 2005). The main results showed that the participation rate in MVPA increased during the study period. The increase was not continuous, but showed an upward trend during the years 2001-2005. The study also revealed that boys participated in MVPA more than girls. However, the gender difference decreased during the study period. Age differences in MVPA were rather small and they decreased during the study period. However, perceived intensity of physical activity was higher among older than younger groups, whereas participation in MVPA was more frequent in younger than older groups. The results supported the findings of some previous studies that reported that especially in Finland the vigorous physical activity among young people had consistently increased. This study gave more information about this phenomenon because the study period was longer, 26 years, and the sample also included older, 16- and 18-year-old boys and girls.