Objective: There has been significant weight gain among Western populations during the past few decades, including children and adolescents. The aim of this research was to investigate the possibility of discovering a correlation between the weight of primary school children and their well-being and performance in studies. There was also an interest in determining the weight development of children and adolescents during a period of thirty years and attempting to answer the question whether they were still putting on weight. Methods and materials: The target population of the research project comprised pupils in the 4th, 7th and 10th grades of primary schools within the service area of Akureyri Health Centre during the winter of 2000-2001. The above classes were chosen on the basis of the fact that they all sat the national coordinated examination. Data were collected to discover Body Mass Index (BMI, weight/height_, kg/m_), measure performance in studies (the results in nationally coordinated examinations in Icelandic and arithmetic) and assess well-being by means of a questionnaire (Youth Self Report, YSR), which was only presented to the 7th and 10th grade. Furthermore, BMI was determined for corresponding groups from the school years 1970-71, 1980-81 and 1990-91, using information obtained from school health reports for the pupils in question. Results: This group consisted of 819 pupils and information was available on 568 of those, or about 70%. The weight gain which turned out to have occurred between the years 1970 and 1990 did not occur in the final decade of the 20th century and this applies to all three year groups. No difference was between those living in urban and rural areas. This was found to be the case, both when comparing the average BMI of the year groups and the proportion of children who are above a certain threshold of criteria. Furthermore, it was found that excessive weight/obesity is related to slack study results and a poor state of well-being among pupils in the 10th grade of the primary school. Such a relationship, however, did not exist among students in the 7th grade. Heavier pupils in the 4th grade perform better in their studies than their lighter peers, although this difference is not statistically significant. Conclusions: The unfavourable trend towards weight gain that we have seen in the past may now be changing. Other research pointing in the same direction has not been found, however, and more results are needed before this can be stated as a fact. Weight has a stronger influence on the well-being of adolescents in their upper teens, or towards the end of primary school, than is the case with younger pupils. This research does not explain why, although it may be assumed that the social environment of older children and adolescents is a significant factor in this respect. As the children grow older, a correlation begins to appear between being overweight and having less performance in studies and none of the heavier pupils show excellent school performance.