The aim of this study was to examine the clinical effect of naproxen sodium after knee arthroscopy. The design was randomized, prospective, and double-blind, with a placebo control group. Patients with preoperative synovial reaction were excluded. One hundred twenty consecutive arthroscopies were performed in 77 men and 43 women, median age 34 (range 18-65) years. General anesthesia was used. Ninety-one arthroscopies were operative, whereas 29 were diagnostic. Postoperatively the patients received either 550 mg naproxen sodium twice daily or placebo for 10 days. Patients were given 500 mg acetaminophen to use if needed. Clinical examinations were performed 10 and 20 days postoperation. The results were analyzed using the Mann-Whitney test and chi 2 test. No operative complications were registered. All but two patients completed the study. Six patients had slight gastrointestinal symptoms. At 10 days' follow-up, in patients who had undergone operative arthroscopy naproxen sodium demonstrated a statistically significant effect on synovial effusion (p < 0.01), range of motion (p < 0.001), quadriceps strength (p < 0.05), pain (p < 0.001), walking activity (p < 0.05), and use of crutches (p < 0.01). In patients who had undergone diagnostic arthroscopy naproxen sodium demonstrated a beneficial effect on pain (p < 0.01). At 20 days' follow-up, in the operative group naproxen sodium affected synovial effusion (p < 0.05), range of motion (p < 0.01) and pain (p < 0.05), and walking activity (p < 0.05). No effect was seen in the diagnostic group at 20 days' follow-up.