A review of the studies on the etiology of habitual thumbsucking reveals either contradictory or inconclusive results. In this study carried out in Turkey, 50 thumbsuckers, 50 non-thumbsuckers, 250 school children and 312 'problem' children were investigated through interviews, questionnaires and other clinical techniques with their mothers. Among variables studied, were aspects of feeding, onset and incidence of thumbsucking, strength of the sucking drive, sex distribution, educational level and occupation of mothers, parental attitudes toward physical contact with children, mother-child relationships, and particular forms of falling asleep. It was found that thumbsucking was etiologically more related to ways of falling asleep than to other factors. An attempt was made to explain the social, psychological and physiological basis of the etiological significance of the falling asleep stage in habitual thumbsucking.