The aim of this study was to evaluate whether or not dental agenesis is regulated by genes and, in those cases under genetic control, if there were differences between incisors-canines and premolars-molars in the genetic transmission of agenesis.This study was conducted on all patients who presented for a clinical evaluation in the Orthodontic Unit of the Department of Dentistry between 2001 and 2002. All patients were screened for dental agenesis. At the end of the selection process, 17 patients with dental agenesis, 6 to 20 years of age, were questioned about their family dental history for the purpose of constructing family trees and 15 family trees were constructed. The patients were questioned regarding any dental anomalies present. Panoramic radiographs, casts, and dental examinations were performed on the patients. For the patient's relatives, panoramic radiographs and oral histories were generally collected.The most often missing teeth were mandibular second premolars. On the basis of the family trees, the data allowed for the identification of two different groups: 1) 9 families had dental agenesis as a function of genetic transmission and 2) 6 families exhibited dental agenesis as a sporadic condition. In the former group of patients, agenesis primarily involved the maxillary lateral incisors. This appeared to be transmitted as an autosomal dominant characteristic, with variable expression and incomplete penetrance. In these families, both in individuals with and without agenesis, other anomalies were present, such as supernumerary teeth, microdontia, and anomalous teeth. In these cases, agenesis was one of the manifestations of an anomaly of the dental lamina, specifically mild dysplasia. In the latter group of patients, agenesis was a characteristic present only in orthodontic patients. In this group, agenesis most commonly involved the second molars and the second premolars.These findings indicate that agenesis of anterior teeth may depend on genes, while agenesis of posterior teeth is sporadic.