The efficacy of tactile detection of the apical constriction in flared and nonflared root canals was examined in 120 root canals of adult patients. In 68 nonflared (group 1) root canals, a #15 or #20 K-file was used to detect ("feel") the apical constriction. In 52 teeth (group 2), Hedstrom files, Gates Gliden drills #2 to #4, and ultrasonic files were used to enlarge the canal orifice and flare the coronal portion of the root canals before testing the apical constriction. After placing a #15 or #20 file in each root canal, a radiograph was taken, and the distance between the tip of the file and the radiographic apex was measured. The location of the file tip was classified into three categories: (a) within 1 mm short of the radiographic apex; (b) underextended, more than 1 mm short of the radiographic apex; and (c) overextended beyond the radiographic apex. In group 1 (nonflared), 32.3% of the root canals were classified in category a, as compared with 75.0% in group 2 (preflared). Over 26% of the root canals in group 1 and approximately 4% of the canals in group 2 were included in category b. Files inserted in preflared root canals had a significantly lower incidence of overextension than those placed in nonflared canals (21% versus 41%). The ability to determine the apical constriction by tactile sensation was significantly increased when the canals were preflared (p < 0.0001).