This study was aimed at determining the prevalence of dental agenesis in permanent dentition.A group of 112 patients who came to UNAM's Dental School imaging department from January to July 2008 were included in the study. The patients had to be over 13 years old, having had no dental extractions or genetic syndromes. A panoramic radiography was taken of each of them. Sociodemographic variables and family history of dental agenesis were recorded. SPSS 15.0 was used for processing the information. Descriptive statistics were used for determining agenesis frequency; X2 was used for assessing agenesis association with gender, family history and family line.53% of the group being studied were female; mean age was 22+/-4.7. 26 % of the patients presented dental agenesis (21.4% of at least one third molar, 4.5 % presented other tooth agenesis). Both paternal and maternal family lines had similar percentages regarding a background of agenesis. Dental agenesis and sex were independent (X2=0.881; p=0.348). Family history and family line showed a significant association (p<0.001).There was 26% overall dental agenesis. Gender and dental agenesis were independent. A family background of agenesis was significantly associated with dental agenesis.