AIM: The aim of this study was to classify early arrested eruption of primary molars and to analyse and explain the sequelae for the surrounding alveolar bone and the succeeding premolar. DESIGN: The position of the arrested primary molars in the mandible, the height of the local alveolar bone, and the morphology and location of the succeeding premolar were evaluated on radiographs from 29 children. RESULTS: Four groups of arrest from mild to severe with regards to infra-position were categorized (Groups I-IV). Mean ages at the time of referral decreased from Groups I (8 years, 10 months) to Group IV (5 years, 9 months). SEQUELAE: (i) Reduction of alveolar bone height (Groups I-III); (ii) delayed maturity of the succeeding premolar (two-thirds of the cases); (iii) malformation of the succeeding premolar (mainly Groups III and IV); and (iv) ectopically located premolar occlusal to the retained molar (Group IV). CONCLUSIONS: The deeper in the alveolar process a primary molar is retained, the earlier the disturbance in the eruption has occurred, and the greater is the risk of the permanent tooth germ being malformed and malpositioned. It is estimated that the earliest occurrences of arrested eruption of primary molars supposedly occur before the age of 3.