The purpose of this study was to determine whether successful dental therapy (scaling and root planing) depends upon handedness of dentists in right-handed dental chairs (units) Participants were 28 voluntary dentists (14 female and 14 male, ranging in age from 26 to 34 years). Patients (7 female and 7 male with a mean age of 39.6 years) had advanced periodontitis. Handedness was assessed using the Turkish version of the Edinburgh Handedness Inventory. There were two equal groups of dentists: left-handers (7 female, 7 male) and right-handers (7 female, 7 male). Initial examination of patients was performed using a pressure-controlled periodontal probe. During scaling and root planing, the negative interstroke forces were recorded using a piezo-electric receiver, an electronic transducer, and an analogue writer. The results showed that during scaling, mean negative forces reached a mean of 0.74 N in left-handed dentists and 0.52 N in right-handed dentists. During root planing, these forces were 0.86 N in left-handed dentists, and 0.63 N in right-handed dentists. These differences were statistically significant. The right-handed dentists were more successful than the left-handed dentists at scaling and root planing, provided that both of them used the same right-sided dental chairs. The importance of handedness of the dentists was accentuated in dental practice.