The primary purpose of this study was to determine if two sessions of scaling and root planing, one using a "closed" approach and the other using an "open" approach, would remove all calculus from teeth with severe periodontal disease. Fourteen teeth (8 single-rooted, 6 multi-rooted) were treated by closed scaling and root planing with an ultrasonic instrument, and 17 others (10 single-rooted, 7 multi-rooted) were treated with hand instruments. After a healing period of 4 to 8 weeks, anesthesia was secured, periodontal flaps were raised, and the teeth were treated a second time using the same instrumentation as before. The teeth were then extracted and prepared for light microscopic evaluation. Twelve of the 14 teeth treated by ultrasonics and 12 of the 17 treated by hand instruments retained calculus. The two types of instruments had similar scores in the treatment of proximal root surfaces, furcal walls, and furcal summits. Hand instrumentation appeared to be more effective than ultrasonics in removing cementum from proximal surfaces, although this is not based on a formal statistical comparison of the two methods. Neither instrument was effective in removing cementum from the furcal walls or summits. Five randomly selected blocks containing remaining root structure were deparaffinized and prepared for scanning electron microscopy. One hand-instrumented specimen, which had not shown calculus with light microscopy, displayed calculus at the SEM level. All five specimens displayed residual calculus at either the light microscope, the SEM level, or both. The results of this study indicate that complete removal of calculus from a periodontally diseases root surfaces is rare.