The Community Periodontal Index of Treatment Need (CPITN) is recommended for monitoring the outcome of periodontal treatment, yet to date there is only one published prospective study that has used the index for that purpose. The aim of the present study was to monitor the outcome of periodontal treatment of 42 patients referred to a consultant periodontist in a dental hospital. The CPITN was recorded at baseline by a single trained examiner. Subsequently, the patients were examined independently by the consultant and a treatment plan formulated. The prescribed treatment was carried out, and a record was maintained of the type of treatment provided and the number of visits involved. The CPITN was recorded again approximately 6 months after baseline by the same trained examiner. Full data were obtained for 36 subjects; most (22 patients) had deep pockets (CPITN, code 4) at baseline, while nine had shallow pockets (CPITN, code 3). Patients with deep pockets at baseline required more complex treatment and more visits (mean 6.2 visits) than patients with shallow pockets, calculus or gingival bleeding (mean 3.5 visits). Nineteen of the 22 subjects with deep pockets at baseline showed a reduction in the number of sextants with deep pockets after 6 months; however, in only 11 of these subjects were the number of sextants with deep pockets reduced to zero. In the light of these findings a modified format for summarising such CPITN data is recommended.