Special diets are consumed as a treatment for diseases such as food allergies, diabetes, lactose intolerance, and coeliac disease. Lifestyle-related diets, such as vegetarian, weight-reducing, sports, bodybuilding, and low-carbohydrate diets, can also be called special diets. The aim of this study was to examine the frequency of and reasons for special diet consumption as a therapeutic measure or as a lifestyle-related choice among 12- to 18-year-old Finns. Nationwide cross-sectional data were collected by mailed, self-administered questionnaires in 1999, 2001, 2007, and 2013. The number of respondents varied from 3535 to 8219, and response rates ranged from 38% to 76%. Consumption of special diets, both for therapeutic purposes and as a lifestyle-related choice, showed a significant increasing trend in 12- to 18-year-olds from 1999 to 2013 (p<.001). During the study period, the most frequently-reported single diets in adolescents were vegetarian and lactose intolerance diets. However, a fluctuation trend was observed in the consumption of different lifestyle-related diets. Further, a growing number of respondents, especially girls, reported simultaneous consumption of a combination of two or more therapeutic and/or lifestyle-related diets. The reason(s) for choosing a therapeutic diet seems to stay constant during the studied period. In contrast, a wide variety of reasons such as health, wellbeing, body weight, ethical, and ideological reasons, and the avoidance of some specific foods were given for the consumption of lifestyle-related diets. Our study results correspond well to the increased prevalence of common illnesses in Finland. Data also reveal the properties of special diet consumption among adolescents in the long run, and this highlights the need to further educate Finnish adolescents on nutrition literacy.