The study compared three populations from different ethnic and socio-geographic groups: Spanish from Galicia, Spanish from Ceuta, and Magrebian from Ceuta. Demirjian stages of tooth maturation were determined for all third molars. The results suggest that Dermirjian stage for tooth 38 can be considered a good indicator of age in all three populations. Differences in tooth 38 mineralization may be related more with socio-geographical than ethnic origin, and constitutes evidence of the relevance of socio-geographic, rather than genetic factors in third molar development. Differences in maturation patterns were found between sexes and populations of origin. Mineralization of tooth 38 was more advanced in males than females among subjects 18 years of age and older in all three populations. Moreover, mineralization of tooth 38 in subjects aged 18 year and older was significantly slower in the Spanish-Galicia population than in the Magrebian-Ceuta population. No significant differences between Magrebian and Spanish individuals from Ceuta were found. The reliability of the method was verified by testing inter-observer and intra-observer agreement. Among methods for estimating the chronological age of the subjects around the age of 18 years, analysis of the stage of third molar mineralization has been proposed because of the absence of other reliable biological markers during late adolescence. Tooth mineralization is evaluated in an orthopantomogram, and dental age is estimated by comparing the dental mineralization status in a person of known or unknown chronological age with dental developmental surveys--standard charts compiled from a large number of persons of known age and in a well-defined geographic region. Tables, figures, references