The aim of this study was to examine the fidelity and completeness of the implementation of a school-based healthful diet promotion programme called 'Krachtvoer' (we use the Dutch title of the programme throughout this article. A possible translation is Power Food, but this does not reflect the play on words in the Dutch title), aimed at lower vocational training students aged 12-14 years. The teachers' and students' opinions of the programme were also assessed, as well as the association between these opinions and the level of implementation. Data were collected through structured teachers' monitoring reports, in-class observations and in-depth interviews with teachers. Further data were drawn from a student questionnaire included in a concurrent cluster-randomized baseline post-test experimental study evaluating the effects of the programme. The present study indicates that the teachers implemented the programme largely according to plan, that the teachers appreciated most parts of the programme and that the students who were exposed to the programme were more appreciative of their nutrition education lessons than students who followed the usual curriculum. However, programme elements that teachers did not like were often not implemented. We conclude that the Krachtvoer programme is a promising tool to encourage students in lower vocational training to adopt diets that are more healthful. The present study revealed the stronger and weaker parts of the programme and can thus guide programme improvement.