Nonprescription doses of naproxen sodium, acetaminophen, and placebo were compared to determine their efficacy and safety in osteoarthritis of the knee. In two identical multicenter, randomized, double-blind, placebo-controlled, multidose, parallel-design studies, patients with osteoarthritis aged (mean ± SD) 60.6 ± 12.8 years were randomized to daily doses of 660 mg naproxen sodium (440 mg naproxen sodium in patients ≥65 years), 4000 mg acetaminophen, or placebo for 7 days. Naproxen sodium (440/660 mg) provided significantly greater improvements in pain at rest, on passive motion, on weight-bearing, stiffness after rest (morning), day and night pain compared with placebo, and significantly greater relief from resting pain than acetaminophen (P < 0.05). Acetaminophen provided significantly greater improvements in day pain compared with placebo. Daily evaluations showed naproxen sodium (440/660 mg) provided superior pain relief to acetaminophen and was significantly better than acetaminophen at reducing difficulties experienced in walking several blocks and difficulties in bending, lifting, and stooping. Naproxen sodium (440/660 mg) and acetaminophen (4000 mg) were significantly more effective than placebo in improving mobility level, household tasks, and walking and bending. Patient and investigator evaluation scores were significantly higher in naproxen sodium and acetaminophen groups compared with placebo; no differences were observed between active treatments. Naproxen sodium and acetaminophen had similar safety profiles to placebo. Nonprescription doses of naproxen sodium (440/660 mg) effectively relieve pain and other symptoms of osteoarthritis. Naproxen sodium is an alternative in the initial treatment of osteoarthritis and may be preferred to acetaminophen as first-line therapy in patients with moderate or severe pain.