The impact of the social environment on food and fat intake was investigated in several samples including family members, close friends, and meal time companions in the Netherlands. Firstly, a food frequency questionnaire to assess the intake of fat, fatty acids and cholesterol was developed. Biomarker-based validity (n = 99), relative validity against a dietary history (n = 191), and reproducibility (n = 93) were satisfactory for adults.Up to 40% of the variance in fat intake, expressed as % of energy intake, could be explained by influences from the social environment. This maximum value was derived from resemblances in fat intake within social networks, including nuclear family members and friends. Associations in fat intake in 1077 nuclear families, based on two-day diet records, ranged between 0.50 and 0.60 between spouses and between siblings, and around 0.40 between parents and children. Resemblance in fat intake was high for food consumed at home and weak for food consumed elsewhere. In another study, fat intake in the last four weeks was assessed with the food frequency questionnaire in 361 social networks, consisting of at most six subjects: a 15 year old adolescent; father; mother; and their best friends. Fat intake was significantly related between nuclear family members (r = 0. 18 to r = 0. 30), but not between best friends.Specific influences from the social environment were quantified: the subjective norm; modelling; and social facilitation. The subjective norm, reflecting the perceived pressure from the social environment, explained 1 % of the variance in fat intake of adolescents (n = 122). The subjective norm appeared to influence fat intake in this group by modifying attitudes towards fat intake. Modelling behaviour explained 6% of the variance in fat intake of adolescents. Mothers were more influential than fathers and best friends. Social facilitation of dietary intake explained up to 5% of the variance in energy intake in young adults (n = 50): spontaneous meal size increased when more ...