Samples of gingival crevicular fluid (GCF) were harvested from sites manifesting features characteristic of active disease including inflammation, periodontal attachment loss, and radiographic signs of alveolar bone destruction in untreated patients with advanced periodontitis. The presence and concentrations of interleukin-1 alpha (IL-1 alpha) and interleukin-1 beta (IL-1 beta) were measured using ELISAs specific for these cytokine molecules. IL-1 alpha and/or IL-1 beta were identified in the GCF of 15 of 15 patients having untreated periodontitis. Ninety percent (71 of 79) of the sites tested contained measureable amounts of IL-1, with IL-1 beta as the more frequently occurring form. IL-1 alpha levels ranged from 0.23 nM to 13.9 nM in the GCFs. IL-1 beta levels were between 0.04 nM and 5.28 nM. Marked reductions of total IL-1 levels were observed following effective treatment. Both forms of IL-1 messenger RNA (mRNA) were detected in 17 of 17 gingival tissue samples from 6 patients. These results demonstrate that IL-1 is produced and released locally in periodontal disease at concentrations sufficient to mediate tissue inflammation and bone resorption. IL-1 may serve as a marker of periodontal tissue destruction.