Objective: The purpose of this pilot study is to examine the relationship between the Periodontal Index (Russell) and full mouth 6 sites per tooth measurements using the criteria used by NIDCR to assess periodontal destruction. Methods: A convenience sample of 22 adults (16 Male/6 Female), age 30 to 72 (mean 41.9, ±2.7), with ≥20 teeth and no prophylaxis during previous 12 months was used. The NIDCR (Miller et al. 1987) diagnostic criteria and instruments were used to conduct full mouth examinations of gingival bleeding, dental calculus, pocket depth (probe depth) and attachment loss. Descriptive statistics, correlational analyses and tests of agreement between the two methods of measuring periodontal disease were conducted. Results: Clinically the group mean Periodontal Index (PI) score (2.07, SE 0.14) equated to “beginning destructive periodontal disease” which was similar to the clinical diagnosis of localized mild to moderate chronic periodontitis based on six sites per tooth. Pearson correlation coefficients between the PI and Calculus (r=0.7693, p<0.001) and Marginal Bleeding (r=0.5163, p= 0.0129) were meaningful and statistically significant. Correlations between the PI and probe depth (r=0.2508, ns) and attachment loss (AL) (r=0.1753, ns) were weak. The percentage of agreement between the PI scores and pocket depth and attachment loss ≥4 mm were poor with Kappa coefficients ranging from -0.0669 to 0.0199 respectively. For pocket depth (< and ≥4mm), observed agreement was 30.0% (p=0.9930); and for attachment loss, 38.7% (p=0.1942). The PI underestimated the frequency of periodontal pockets ≥4 mm by 63.2% (CI 59.3 to 67.1%) and overestimated the frequency < 4 mm by 6.8% (CI 4.8 to 8.8%). A similar pattern existed for attachment loss. Conclusions: In this sample of adults with clinically mild to moderate chronic periodontitis, the PI significantly underestimated the presence of periodontal pockets ≥4 mm.