20 chronic periodontitis patients were given a full periodontal examination, including measurements of probing depth, clinical attachment loss, gingival index, bleeding index and plaque index. At a second visit, gingival crevicular fluid (GCF) was collected from the deepest accessible probing site of each tooth. The patients then received scaling, root planing and other appropriate nonsurgical treatment. GCF was collected from the same sites as sampled pretreatment and clinical parameters were measured again. Cathepsin B/L-, elastase-, tryptase-, trypsin-, and dipeptidyl peptidase IV-like activities in GCF samples were determined by fluorimetric assay with peptidyl derivatives of 7-amino-4-trifluoromethyl coumarin. Following treatment, there were reductions in all clinical parameters and all protease activities. Most were statistically significant both on a patient level using average patient values and on a site level using either individual patient or pooled patient data. As in previous pre-treatment comparisons, post-treatment protease levels correlated positively and significantly with the corresponding clinical parameters at patient and site levels. The reductions and correlations were more marked for total enzyme activities than concentrations. GCF protease levels appear to reflect the clinical status of periodontal lesions and may thus be of value in monitoring disease activity.