The authors conducted a prospective, randomized, crossover study of positive aspiration frequency in intraligamentous anesthesia (ILA). The aims of the study were (1) to determine the frequency of positive aspiration in ILA and (2) to evaluate the depth of the pulp anesthesia in lower first molar (L6) by means of electrotesting in positive or negative aspiration. Intraligamentous injections with 4% articaine with epinephrine in L6 were carried out with computer syringe in 36 dental students. Anesthesia of these 36 molars was obtained in 114 injections, with 3.2 injections on average. Each injection was controlled by electrotesting. During each ILA the last 1-2 injections were accompanied by positive aspiration and simultaneously by pulpal analgesia (testing values 100-200 mka). In 34 of 36 (94.4%) subjects intraligamentous anesthesia was accompanied by positive aspiration test (AT). In 2 cases (2/36/5.6%) successful but slowed anesthesia had negative AT in all 5 injections. The phenomenon of aspiration in ILA was studied for the first time. Positive aspiration (94.4%) is a precursor of effective anesthesia. This phenomenon is presented as an evidence of vascular mechanism of ILA.