Eating and sleeping problems have a high prevalence in mental retardation in general, but are also discussed as characteristic in some genetically determined disorders. A comparative analysis of eating and sleeping behaviours in 28 Prader-Willi- and 32 Williams-Beuren syndrome children by psychometric instruments confirms excessive food-seeking behaviours in PWS and selective food refusal in WBS as specific problems. In both syndromes, however, there is considerable individual variability in these symptoms.