The purpose of this analysis was to assess the ability of the CPITN system to rate severity and prevalence of periodontal diseases in a population, in comparison with full mouth scorings using conventional clinical parameters (Plaque Index, Gingival Index, Retention Index, Pocket Probing Depth and Loss of Attachment). These parameters were collected in a randomly selected sample in Switzerland. The data were then transformed to fit the definitions of the CPITN. Furthermore, a comparison of the data set from Switzerland with data obtained from the Oral Global Data Bank of the WHO was made. By conversion of the Swiss data into the CPITN format, many details were lost, which were considered to be relevant to assess severity, prevalence and localization of periodontal diseases within a population. In addition, the transformed data generally overestimated the prevalence of periodontal destruction when compared with data from surveys in other industrialized countries in which the CPITN was used to evaluate the periodontal status. The comparison of the data before transformation into the CPITN, however, corroborated results from epidemiological studies in which conventional periodontal index systems were used. This indicates that data obtained to determine defined treatment needs (CPITN) may be of questionable value for the assessment of the true prevalence and severity of periodontal disease in a population.