The purposes of this study were to: 1) characterize the demographics, oral health behavior, and periodontal status of a health maintenance organization sample; 2) investigate the relationship between the location of posterior proximal measurement sites and prevalence estimates for periodontal disease; and 3) compare the prevalence of persons with pockets > or = 4 mm in the present sample to the 1985 NIDR Survey of Oral Health. The sample consisted of 1,090 adults attending a large health maintenance organization. All proximal sites in one randomly selected posterior dental sextant were examined for probing depth using a constant force probe. Demographic, medical, and behavioral factors were determined by questionnaire. Results indicated that the sample consisted primarily of medically and periodontally-healthy middle-aged adults with good oral hygiene habits. Overall, the mean probing depth was 2.95 mm with 10.1% of sites/subject > or = 4 mm. A larger percent of subjects had probing depths with pockets > or = 4 mm at lingual proximal sites than buccal proximal sites. Prevalence of subjects with pockets > or = 4 mm at mesio-buccal sites in the present study was similar to NIDR Region III data (15.3% vs. 17.4% respectively). However, when data from all posterior sites were included, the overall prevalence rate in the present sample increased to 36.8%. These findings indicate that disease prevalence is dependent on the location of surfaces measured and conservatively indicate that NIDR survey data may have underestimated the prevalence of persons with periodontal pockets > or = 4 mm by at least 20%.