The purpose of the present study was to subclassify by phenotype those patients who had retained first molars. Radiographs from 53 male and 37 female patients (7 years, 10 months to 17 years, 8 months) comprised the study sample. Subdivision of material according to primary retention (arrested eruption of the first molar occurred before the tooth penetrates the oral mucosa) or secondary retention (continued eruption of a molar arrests after the tooth has penetrated the oral mucosa) revealed that the average age at first diagnosis of primary retention was 9.2 years and of secondary retention 13.8 years of age. The study indicated that primary retention was more frequent than secondary retention and that male and female patients were affected with equal frequency. Primary retention occurred more often in the maxilla than in the mandible, whereas the opposite was found in secondary retention. Primary retention can occur in dentitions that have normal development of the second molar with or without generalized malocclusion, as well as in dentitions with abnormal eruption patterns in the second molar region (late development and second molar agenesis), with or without generalized malocclusion. Secondary retention can occur in dentitions with normal development of the second molar with or without generalized malocclusion. The phenotypic differences noted between the primary and secondary retention was that variations of normal eruption of the second molar were seen in the cases with primary retention. This observation indicates that primary retention can be an acquired condition.