Objective: Mechanical instrumentation is fundamental to periodontal treatment. However, independent of the instrument used in scaling, an immediate attachment loss occurs at the bottom of the periodontal pocket. This study aimed to determine the influence of tip diameter on attachment loss and the influence of attachment loss on the periodontal response to nonsurgical treatment. Method and Materials: Fifteen patients presenting periodontal pockets with a probing depth of 3.5 mm or more in bilateral teeth were divided into two groups: test group-instrumented with a thin tip, and control group-instrumented with a traditional tip. Probing depth, relative gingival position, and relative attachment level were evaluated immediately before and after and at 1 and 3 months after treatment using an electronic computerized probe. The data were analyzed using ANOVA and Tukey tests (P = .05). Results: Both groups presented attachment loss immediately after instrumentation; however, the thin tip resulted in statistically higher immediate clinical attachment loss than the traditional tip (0.85 and 0.15 mm, respectively; P .0001). However, both groups showed a similar healing after the ultrasonic therapy, at all parameters evaluated (P > .05). Conclusion: In spite of the higher immediate clinical attachment loss inflicted by thin ultrasonic tips during instrumentation, this did not affect the clinical response to the nonsurgical treatment. Keywords: adverse effects, dental scaling, periodontitis, randomized clinical trial, thin tips, ultrasonic device