Scaling and root planing are the most common techniques utilized to achieve a biologically-acceptable root surface. Thorough root debridement is a demanding task, with residual deposits of plaque and calculus a not uncommon finding after instrumentation. This study evaluated the effectiveness of scaling and root planing via a closed versus an open flap approach. Sixty multi-rooted teeth were assigned to one of three groups: untreated controls, closed scaling/root planing, and open flap scaling/root planing. Following debridement, teeth were extracted, immersed in methylene blue, and examined for the percent surface area having stainable residual deposits. The mean percent stained surface area covered by residual plaque and calculus was 54.3% in the closed root planing group compared to 33.0% in the open flap root planing group. The untreated control teeth had 91.0% of the root surface covered with stainable deposits. Within-group comparisons showed no significant difference in the percent stained residual plaque and calculus in shallow areas of the pocket (< or = 3 mm apical to the gingival margin) compared to deeper areas (> 3 mm subgingival). Examination of furcation regions demonstrated heavy residual stainable deposits for both treatment methods, with no significant differences between techniques. There was no correlation between the time spent in root debridement and the percent residual deposit area. The results demonstrate that hand instrumentation alone is inadequate for thorough debridement of furcations and suggest that new approaches are needed to provide a root surface which is compatible with formation of new periodontal attachment. High frequency ultrasonic instruments, rotary burs, and chemical agents may assist in debridement of such surfaces.(ABSTRACT TRUNCATED AT 250 WORDS)