In this double-blind crossover trial, naproxen (750 mg/day) was compared to placebo for the treatment of osteoarthrosis of the hip and knee. Patients were randomly assigned to treatment with either naproxen or placebo for 4 weeks and then to treatment with the alternate agent for a second 4-week period. 8 out of 9 objective and subjective measurements of drug efficacy clearly differentiated naproxen from placebo at highly significant levels (P = 0.0001 to 0.0004). Patient daily check lists of osteoarthrotic symptoms also showed a statistically significant difference between naproxen and placebo therapy. Both physicians and patients, when asked to give a 'final drug preference', showed a significant preference for naproxen over placebo. In general, the incidence of side effects was low and approximately the same for both naproxen and placebo. Laboratory assessments showed little difference between groups. The trial showed naproxen to be an effective and well tolerated drug for the treatment of osteoarthrosis of the hip and knee.