The prevalence of chronic periodontal disease was investigated using the occurrence of diseased pocket sites as the diagnostic criterion. The study group comprised 250 dentally aware subjects aged 21-60 years, with a high number of teeth remaining in all age groups. The % of sites with a probing depth of greater than or equal to 4 mm was 16% for the total sample. Most diseased sites (69%) were in the 4-mm class, with only 4% at depths greater than or equal to 7 mm. 82% on the subjects had 1 or more sites greater than or equal to 4 mm. With probing depth as the sole criterion for diagnosis of chronic periodontal disease, prevalence in a sample is heavily dependent on the critical levels selected for probing depth and cut-off point. With critical levels of 4 mm for probing depth and 1 diseased site for the cutoff point, prevalence was 49% in the 21-30 year age group and 95% in the 51-60 year age group. A shift in cutoff point to 40 diseased sites gave a prevalence of 2% and 41%, respectively, in the 2 age groups. For a critical depth of 7 mm and a cutoff point of 1 diseased site, the prevalence was 2% and 43% for the 2 age groups, respectively. Aspects of importance for the decision-making process on disease are discussed.