Diogenes is a Pan-European, randomized, controlled dietary intervention study investigating the effects of dietary protein and glycaemic index on weight (re)gain, metabolic and cardiovascular risk factors in obese and overweight families in eight European centres. The article is methodological in character, and the presentation of 'results' will be limited to baseline characteristics of the study populations included. A total of 891 families with at least one overweight/obese parent underwent screening. The parents started an initial 8-week low-calorie diet and families with minimum one parent attaining a weight loss of > or = 8%, were randomized to one of five energy ad libitum, low-fat (25-30 E%) diets for 6 or 12 months: low protein/low glycaemic index, low protein/high glycaemic index, high protein/low glycaemic index, high protein/high glycaemic index or control (national dietary guidelines). At two centres the families were provided dietary instruction plus free foods for 6 months followed by 6-month dietary instruction only. At the remaining six centres the families received dietary instruction only for 6 months. The median weight loss during the low-calorie diet was 10.3 kg (inter-quartile range: 8.7-12.8 kg, n = 775). A total of 773 adults and 784 children were randomized to the 6-month weight (re)gain prevention phase. Despite major cultural and dietary regional differences in Europe, interventions addressing effects of dietary factors are feasible with a reasonable attrition.