The investigation was carried out for the purpose of establishing the frequency of thumbsucking and the importance of the sucking factor for the etiology. Thumbsucking and other comparable sucking occurred at the age level of one year in 50% of the cases. At the age of four years there was a frequency of 36%. These figures include both the mild and the more severe degrees of thumbsucking. If only the latter are taken into account the figure for age four is about 14%. One of the primary determinant factors in the arisal of the habit, as far as can be judged, is the degree of satiation in infants of the sucking activity. The probability of this appeared from comparisons of the mode of feeding. In any event, none of the at least four year old children who had used a dummy teat (rather more than 10% of the entire series) had ever been thumbsuckers.