The present investigation was carried out to study the prevalence and clinical appearance of destructive periodontal disease in a school population of 15- to 16-year old adolescents in Amsterdam. In addition the prevalence of Actinobacillus actinomycetemcomitans was studied in those subjects showing attachment loss. Attachment loss was diagnosed in 230 of the 4565 subjects participating which is about 5% of the population studied. Within this population males were more frequently affected than females (P = 0.008). Extensive periodontal destruction was found in 16 subjects (0.3%). In this group a female/male ratio was found of 1.3:1. As a result of the epidemiological survey, 105 subjects with attachment loss volunteered for further investigation. The results showed that presence of plaque, redness and swelling of the gingiva, and bleeding on probing were general phenomena. In addition, the bleeding/plaque ratio, as determined for each subject at sites without attachment loss, increased with the severity of periodontal disease as expressed by the number of sites with attachment loss (P = 0.0038) as well as by the amount of destruction at these sites (P = 0.002). A. actinomycetemcomitans could be identified in 18 of the 105 subjects with attachment loss. Analysis showed that A. actinomycetemcomitans was more frequently isolated in subjects with moderate to severe periodontal breakdown than in subjects with mild breakdown (P less than 0.02). When estimating the percentage of juvenile periodontitis patients on the basis of the classically accepted criteria, it seems likely that between 0.1% to 0.2% of the population in Amsterdam is suffering from this disease entity.