Radiographs of 2550 orthodontic patients were evaluated for tooth agenesia dependent on the presence and the missing of third molars. Tooth agenesia (except third molars) was found in 248 patients (9.7%). The missing of third molars occurs at 427 of 2061 patients (20.7%) aged between 12 and 36 years. At 81.5% of all patients with tooth agenesia at least one of the third molars was missing. The rate of agenesia had an amount of 5.9% (97 of 1634 patients) in the group of patients with presence of all third molars. Whereas the rate of agenesia at patients with missing of one or more third molars was with 27.6% especially great. Moreover it was found out, that the rate of agenesia was the greater the more third molars were missing.