The aim of the present study was to assess a possible association between radiographic bone loss and presence of Actinobacillus actinomycetemcomitans on the site and subject level over time. Forty-six teenagers with registered radiographic bone loss and 8 controls participated in the study, which began in 1984. In 1986 and 1988 new sets of bite-wing radiographs were obtained and examined for bone loss. Subgingival plaque was sampled from both diseased and contralateral healthy sites in the same individual. Only 15 of the 46 originally diseased sites persisted with bone loss in 1986, and 60% of these sites harbored A. actinomycetemcomitans. Of the 31 "healed" sites, 29% contained A. actinomycetemcomitans. The difference was statistically significant at P less than 0.05. Six of the contralateral healthy sites had developed bone loss during these 2 years and 50% of them contained A. actinomycetemcomitans, while 35% of the remaining contralateral sites harbored A. actinomycetemcomitans. The difference was not statistically significant (P greater than 0.05). The difference between patients with and without persistent bone loss regarding presence of A. actinomycetemcomitans was statistically significant in 1986; however, none of the observed differences were statistically significant in 1988. A. actinomycetemcomitans was found 2 to 3 times more frequently in patients with radiographic bone loss at all examinations than in "healthy" subjects. Detection of A. actinomycetemcomitans on the site level appeared to be a poor predictor of future radiographic bone loss.