Objective: Salivary enzymes may be used to diagnose periodontal conditions. Salivary arginase activity (SAA) is related to susceptibility to bacterial infection. Therefore, the aim of this controlled interventional study was to determine the SAA before and after nonsurgical periodontal therapy. Method and Materials: Eighty-nine subjects were selected: 31 periodontally healthy patients (controls), 27 gingivitis patients, and 31 chronic periodontitis patients. Plaque and Gingival Indices, probing depth, and clinical attachment level were monitored. The presence of Campylobacter rectus, Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola, and Prevotella intermedia was evaluated by polymerase chain reaction. Salivary total protein level and SAA were also established by spectrophotometry. Clinical and arginase data were analyzed using the Wilcoxon, Mann-Withney, and Kruskal-Wallis tests (P .05). For microbial data, the chi-square test was used. The Pearson correlation test was also used between each parameter evaluated. Results: After therapy, due to a significant reduction in SAA, the values observed for the gingivitis and periodontitis groups were similar to those found in the healthy group. Interestingly, after therapy, SAA followed the same positive pattern showed by the overall improvement of clinical parameters (gingivitis and periodontitis groups mean values, pre- > posttherapy) and by the reduction of target pathogens (gingivitis group T forsythia, pre- > posttherapy; periodontitis group P gingivalis, T denticola, P intermedia, and T forsythia, pre- > posttherapy). Conclusion: Based on the reduction of SAA after therapy, in accordance with the expected reduction in clinical and microbiologic parameters, it was concluded that SAA has the potential to serve as a reliable method to access to the therapeutic response of chronic periodontitis subjects treated with nonsurgical periodontal therapy. Keywords: arginase, gingivitis, periodontitis, therapy