Objective: To examine under- and over-reporting of energy intake (EI) among adolescents and to compare relative food and nutrient intakes of under-reporters (UR), over-reporters (OR), and the whole population to acceptable reporters (AR). Design: All adolescents completed food frequency questionnaires at regional health centers, and a subgroup also completed a 3-day weighed dietary protocol at home. Setting: This study is a part of the first national representative study on dietary habits of Slovenian adolescents. Participants: In total, 2,813 adolescents entering high school (10% of the population); participation rate was 95%. Main Outcome Measures: Absolute EI and relative intakes of food and nutrients. Analysis: Prodi (version 5.2 expert plus, Nutri-Science, Stuttgart, Germany, 2004) software was used to evaluate dietary intakes; t test, analysis of variance, and Mann-Whitney testing were used for differences between means, and chi-square was used for differences between proportions. Level of significance was set at "P" = 0.05. Results: The prevalence of UR and OR was 34% and 10% among boys, and 27% and 11% among girls. Under-reporters reported lower energy percentage from carbohydrates, higher energy percentage from fats and proteins, and higher micronutrient densities than AR. Conclusions and Implications: Under-reporting and over-reporting are widespread among Slovenian adolescents. Exclusion of UR and OR does not influence mean value of EI when assessing the diet of a group as a whole. (Contains 4 tables.)