The purpose of this study was to compare differences in pocket reduction rate between different tooth types and sites after initial treatment. Forty-one adult periodontitis patients (mean age 40.8) whose O'Leary plaque records during initial treatment were maintained at a level of less than 10% (mean level 9.02 +/- 4.93%) were selected for this study. A total of 5983 surfaces were monitored by recording probing pocket depths (P.P.D.). The results obtained were as follows. 1. Significant probing pocket depth reduction was obtained following initial treatment (P less than 0.001). 2. Significantly improved teeth were 421/124 (p less than 0.001), 5/5 and 53/35 (p less than 0.05), and 32/23 (p less than 0.05). Deep residual P.P.D. were observed in 1/1, 6/6, 7/7, 6/6 and 7/7 (p less than 0.001), and 2/2 and 3/3 (p less than 0.05). 3. Response to treatment by tooth type was favorable in 54/45 and 431/134 and was poor in 71/17, 7/7. 4. Mandibular residual pocket depths were much deeper on proximal surfaces than on the mid-facial and lingual surfaces. Maxillary pockets on palatal surfaces were significant deeper than on facial ones. 5. Poor response to treatment was treatment was observed on the mesio-palatal surfaces (p less than 0.001), mid-palatal surfaces (p less than 0.01) of 6/6 (p less than 0.001), the distolingual surfaces of 7/7, and 7/7 (p less than 0.001), and the mesio-palatal surfaces of 1/1 (p less than 0.05). Favorable responses were observed on the disto-buccal and the id-palatal surface of 4/4 (p less than 0.05), mesio-distal buccal surfaces of 5/5 (p less than 0.05) and mesio-lingual surfaces of 4/4 (p less than 0.05). This results clearly indicate which tooth types and root surfaces must be paid careful attention to during treatment.