Active and inactive periodontal pockets exist in periodontal disease and the progression of periodontitis is episodic and cyclical. Current diagnostic tests do not distinguish between active and inactive lesions. Objective assessment of disease activity could significantly affect periodontal therapy. Aspartat aminotransferase (AST) activity in gingival crevicular fluid is a potential quantitative marker of periodontal disease activity. Thirty-six patients with periodontitis, twenty with adult periodontitis and sixteen with rapidly progressive periodontitis were evaluated clinically prior to treatment and AST activity in periodontal pockets was determined prior to and after initial therapy. Clinical measures included plaque index, gingival inflammation degree and attachment loss. The results show that AST levels do not correlate with clinical indices and that they decrease after treatment. AST is a possible novel biochemical marker of periodontal disease activity independent of commonly used clinical measures. It could also be useful for early monitoring of treatment success.