Previous studies have shown that silicone oil (dimethylsiloxane) and organic oils have a plaque-inhibiting activity, when they are used for oral rinsing. To determine whether organic oils would also influence the inflammatory reaction in periodontitis, we performed a double blind clinical study using soy oil for subgingival irrigation prior to scaling. Twenty patients with at least 3 teeth (test-teeth) in the upper jaw with probing pocket depth >= 5mm were randomly divided into 2 groups. The following clinical parameters were assessed: A modified plaque index (PLI) at 4 sites/tooth, pocket depth (PD), attachment level (AL) and bleeding upon probing (BOP) at 6 sites/tooth. All measurements were performed with an electronic probe (Florida Probe®). Prefabricated acrylic stents were used for AL measurements. Two weeks after repeated supragingival prophylaxis, patients were analyzed at baseline, days 1, 3, 15, 28 and 90. All teeth with PD >= 5 mm were irrigated with either 1 ml soy oil (99%) (Test group = Tg) or NaCl (Control group = Cg). Our preliminary results showed a reduction in BOP at the test teeth after day 15 in the two groups (BOP=from 38.3% to 28.8% at Tg, and from 41.6% to 27.2% at Cg). This is probably due to the reduction in PLI from approximate 40% before baseline to almost 15% in both groups and indicates the therapeutic effect of subgingival irrigation irrespective of the solution used. PD and AL showed a (non-significant) reduction in both groups. We conclude that the use of soy oil for subgingival irrigation prior to root planing, doesn't markedly influence clinical signs of inflammation. Keywords: periodontitis, subgingival irrigation, oils