The beneficial effects of comprehensive dental prophylaxis programs are well recognized. In order to study the factors determining the effectiveness of these programs, 28 periodontal recall patients were subjected to monthly professional mechanical tooth cleanings. Experimental and control teeth were randomly selected according to a splitmouth cross-over method. The Plaque and Gingival indices were recorded 1 month prior to the study, at the start, and at 4, 8, and 12 months. One month prior to the study calculus and bacterial plaque were thoroughly removed from all teeth. During the study period only the experimental teeth were cleaned. Efforts were made not to influence the oral hygiene of the participants by any other means. Following the initial tooth cleaning and during the preexperimental period the Plaque and Gingival indices decreased to a low level on both the experimental and control teeth (mean P1I = 0.4, mean GI = 0.3). This high standard of oral hygiene and gingival health was maintained throughout the study period, and no differences could be observed between experimental and control teeth. This indicates that factors other than the mechanical professional cleaning were responsible for the maintenance of gingival health. It is suggested that participation in the program in itself may have motivated the patients to improve their oral home care.