AbstractNinety‐five patients with periodontal disease each had subgingival plaque samples collected from four sites (one from each quadrant) in their mouths. The relative proportions of spirochaetes, motile rods and cocci were determined using dark field microscopy and the proportion of anaerobic to aerobic microorganisms calculated after culture. In addition, clinical recordings were made.The only significant correlations observed were between probing depth or attachment loss and the proportion of cocci in the plaque (negative association), probing depth or attachment loss and sites which were suppurating or displayed a radiolucent interdental crest (positive association), and the percentage of cocci and sites with a radiolucent interdental crest (negative association). Partial correlation analysis controlling for probing depth or attachment loss showed that a significant inverse association between the percentages of cocci and the presence of a radiolucent interdental crest remained. No significant associations were observed between clinical signs such as bleeding on probing or suppuration and the microbiological assays. Overall there was a poor correlation between many of the signs thought by some to indicate periodontal disease activit