This study evaluated the ability of clinicians to detect residual calculus following subgingival scaling and root planing and compared the clinical detection to the microscopic presence and surface area occupied by calculus found on teeth extracted after instrumentation. Interexaminer and intraexaminer reproducibility in clinically detecting subgingival calculus was also determined. One hundred one extracted teeth with 476 instrumented tooth surfaces were evaluated stereomicroscopically for the presence of calculus and the percent surface area with calculus was determined by computerized imaging analysis; 57% of all surfaces had residual microscopic calculus and the mean percent calculus per surface area was 3.1% (0 to 31.9%). Shallow sites had greater surface area of calculus than moderate and deep sites. The difference was not significant. The interexaminer and intraexaminer clinical agreement in detecting calculus was low. There was a high false negative response (77.4% of the surfaces with microscopic calculus were clinically scored as being free of calculus) and a low false positive response (11.8% of the surfaces microscopically free of calculus were clinically determined to have calculus). This study indicates the difficulties in clinically determining the thoroughness of subgingival instrumentation.