Primary impaction of primary teeth is rare. Only ten cases have been found in the world literature. It affects almost always second molars and the finding is usually occasional. Three cases of this abnormality, two of the upper jaw and one of the mandible, are presented. The analysis of the characteristics of these cases and the cases in the literature, suggests that ankylosis has probably a leading role in the etiopathogenesis. The following criteria for a correct diagnosis is presented: 1) deep retention into the bone; 2) absence of caries, restorations and resorptions; 3) frequent passing of the corresponding permanent premolar; and possible retention of the corresponding adjacent tooth. More attention is recommended in the study of the lesion for the future.