The morphological composition of the subgingival plaque associated with active periodontitis lesions and comparable inactive control lesions were determined in a 36-year-old patient with advanced periodontal disease. Monthly measurements of the clinical attachment level were subjected to linear regression analysis as a function of time. Attachment loss was recorded during the monitoring period at 10 sites, with loss of 2 mm or more at 6 sites. Plaque samples taken from these 6 sites and from 6 control sites with similar clinical conditions were not of uniform composition. Active lesions harboured on average 46% more cocci and twice as many large spirochetes as inactive control lesions. A linear regression analysis of the proportions of different morphotypes in the subgingival plaque versus the calculated monthly attachment loss rate at the time of sampling provided only very weak correlations, if at all. The observations made in the patient under investigation suggest that there is no great probability of suspected differences in the floras of active and inactive lesions being detected by dark-field analysis alone. The role of motile rods and spirochetes may have been over-rated hitherto.