Subgingival samples were obtained from mesial and distal surfaces of first molars in 100 subjects aged 14 years at baseline. Sites were monitored at 6-monthly intervals for clinical loss of attachment. Six subjects showed greater than 1 mm loss of attachment at one site 18-24 months later. Further samples were obtained from these sites together with a comparison sample from the contralateral site and equivalent sites in a matched control subject. Actinobacillus actinomycetemcomitans was more frequently isolated from sites following loss of attachment, but it was also found in one comparison site and one control subject. Bacteroides gingivalis was found in only one subject at a site which subsequently experienced loss of attachment. A higher median proportion of spirochaetes was observed in sites after diagnosis of loss of attachment. The median proportion of Bacteroides intermedius was higher at both the baseline and 12-month examinations in subjects who subsequently lost attachment. Further work is required to substantiate the differences which preceded diagnosis of loss of attachment.