We assessed the presence and extent of calculus on subgingival root surfaces of teeth that received scaling and root planing (S/RP) alone, S/RP with modified Widman flap, or no treatment. After extraction, each surface was examined to determine the pocket depth (PD), area of root surface exposed to the pocket (A), and amount of pocket area showing retained calculus (C). Calculus-positive teeth (CPT) and surfaces (CPS) and percentage of pocket area occupied by calculus (C/A) were derived for each group. In general, CPT and CPS were significantly lower after S/RP with flap (37% and 14%, respectively) than after S/RP alone (62% and 24%). The advantage of S/RP with flap was greatest for facial and lingual surfaces and for anterior and premolar teeth. In both treatment groups CPS were similar over a pocket depth range of 0 to 6 mm. But in deeper pockets, CPS in teeth treated by S/RP with flap remained constant at 17% while after S/RP alone CPS increased linearly to approximately 45% at greater than 8 mm. The mean C/A was essentially equal in both treatment groups (11%) and was not related to pocket depth.