Electronic odontometer (Goof, Denmark) was used in clinical investigation of 169 root canals of 138 teeth, treated for different endodontic reasons. A removable plastic collar was fixed to endocanal electrode (K-reamer) on which the exact length was signalized as apical constriction, followed by radiographic control. The obtained values were statistically elaborated. The exact location of apical constriction was accurately determined in 119 cases (70.41%). Electroodontometer errors were found in 50 root-canals, and deviation ranged from -4 mm to -1 mm with mean of -0.3254 +/- 0.85. The predominant error was failure of the apical constriction (39 cases); in a few samples (11) a canal instrument passed through this location. When pulpo-periodontal condition was considered the greatest number of errors was found in teeth with vital pulp tissue and a significantly smaller number in teeth with gangrenous pulp with or without periapical lesions. The age of sample had no influence on the accuracy of determination of the location of apical constriction.