In 1979, the Department of Human Nutrition of the Agricultural University at Wageningen began a research project on obesity in young adults. The main objectives of this project are the investigation of aetiological aspects and health consequences of obesity. It is a prospective study and is being carried out in a young adult population of three age groups (19-21, 24-26 and 29-31 years as at 1 January 1980) in the Municipality of Ede. The present thesis deals with the investigations into determinants of body fatness and two methodological studies concerning relative weight and habitual physical activity which form the first part of the research project mentioned above.<p/>In the Introduction, a general survey is given of the methods of measuring body fatness. Health consequences and aetiology of obesity are also surveyed. Approximately 3900 subjects participated in the studies described in Chapter 2 and 3, and a subsample of approximately 300 subjects from the initial population participated in the studies described in Chapters 4-6.<p/>The relationship between various socio-demographic variables and body mass index (BMI; weight/height <sup><font size="-1">2</font></SUP>) is dealt with in Chapter 2. The prevalence of severe obesity (BMI ≥30.0 kg/m <sup><font size="-1">2</font></SUP>) was 2% in both sexes. The most salient socio-demographic determinants of BMI were age and level of education. Older subjects were heavier, and subjects who had a higher level of education were lighter. Other socio-demographic determinants of BMI were: level of father's occupation and religion in both sexes; parity in females; and urbanization and church attendance in males.<p/>The relationship between aspects of daily life-style and BMI are described in Chapter 3. Five conceptually meaningful factors could be distinguished within the aspects of daily life-style considered. These factors were interpreted as constructs of: 1, slimming behaviour; 2, behaviour characterized by the consumption of coffee and alcohol, smoking habits and number of hours of sleep per night (CASS behaviour); 3, eating sweet and savoury snacks; 4, health-conscious behaviour; and 5, physical activity. Heavier subjects had higher ratings for slimming behaviour and lower ratings for health-conscious behaviour. In addition heavier males had higher ratings for CASS behaviour.<p/>Possible improvement in the prediction of body fatness when an index of relative weight is based not only on body weight and body height, but also on frame size, is considered in Chapter 4. The prediction of body fatness was not improved when frame size, as assessed by knee width and wrist width, was taken into account. BMI was the most preferable of the indices considered.<p/>The development of a short questionnaire for the measurement of habitual physical activity is discussed in Chapter 5. Three dimensions could be distinguished within the pattern of habitual physical activity. They are interpreted as: 1, physical activity at work; 2, sport during leisure time; and 3, physical activity during leisure time excluding sport. Level of education was inversely related to physical activity at work, and positively related to physical activity during leisure time excluding sport. Subjective experience of work load was inversely related to both sport and other physical activities during leisure time. Lean body mass was positively related to both physical activity at work and sport, in males only.<p/>The relationship between body fatness and both food consumption and habitual physical activity are dealt with in Chapter 6. The average daily energy intake in males was rather high (about 3000 kcal) and average body weight increased by 1.2 kg in the preceding year. Both findings suggest the existence of a positive energy balance. In females the average daily energy intake of about 2170 kcal was closer to the recommended intake and average body weight remained constant. There were no important differences in the pattern of habitual physical activity between the categories of body fatness defined. However the daily energy intake of fatter subjects was lower than that of leaner subjects. This finding may be explained by a reduction of food intake in fatter subjects in an attempt to reduce body weight. There is also some evidence to suggest that many fatter subjects have a reduced need for energy, but it is not clear whether this is a causal factor of obesity or an effect of prolonged slimming.<p/>Finally, a general discussion of the results is given in Chapter 7. While some of the findings can be useful for the management of obesity. it must be emphasized, that several aspects have yet to be studied in detail in the research project of which this thesis forms the first part.