Recent studies have outlined the role of bisphosphonates, and particularly clodronate, as potential therapeutic agents for inflammatory and degenerative joint diseases. On this basis, we carried out an open, non comparative pilot trial to evaluate the effects of clodronate on synovial fluid concentration of some inflammatory mediators (prostaglandin E2, leukotriene B4 and tromboxane B2) after intra-articular, repeated administrations in 20 patients (7 males and 13 females) with synovitis secondary to knee osteoarthritis. At the end of the treatment period, statistically significant reductions (p < 0.05) of spontaneous pain and pain on active movement, evaluated by means of a 100 mm visual analogue scale (VAS), were reported. Linear regression analysis showed that the decrease of pain was correlated with the bisphosphonate induced reduction of prostaglandin E2 levels. These results, in spite of the limitation due to the open design of the trial suggest a possible role of bisphosphonates in the treatment of knee osteoarthritis.