In a controlled study using recently developed and validated methods for eliciting and describing family interactions, a characteristic dysfunctional pattern of interaction was found in families with an obese child. The pattern differed from patterns predicted by previous workers on the basis of indirect evidence or non-systematic study. The pattern was present in all the families studied, but was more marked in the sub-group recruited from a local school, than from subgroups recruited through medical sources. This sub-group had a more positive attitude to obesity and a slightly lower degree of obesity. No common or characteristic interactional pattern was found in the controls. The results were not explainable in terms of demographic criteria, family structure or composition variables, or family emotional health. The findings are discussed in relation to a model of obesity as a family syndrome and a manifestation of psychosocial identity.