A multi-centre randomized, double-blind, parallel-group clinical trial was carried out in 63 patients with osteoarthritis of the knee to compare the efficacy and tolerability of a course of intra-articular injections of 20 mg sodium hyaluronate with a similar course of injections of placebo. Treatment consisted of up to 11 injections over a 23-week period. Evaluation was by means of subjective symptom and activity assessments, serially during the course of treatment and also 25 weeks thereafter. Ten patients (5 of 30 on active treatment; 5 of 33 on placebo) were withdrawn prematurely. Pain on movement, assessed by visual analogue scale (VAS) showed statistically significant (p less than 0.05 to p less than 0.0001) reductions in mean scores throughout the first 11 weeks of treatment with sodium hyaluronate but smaller, non-significant, reductions with placebo treatment. The difference between treatments was significant (p less than 0.05) at 5 weeks. Pain at rest, also assessed by VAS, showed little change in mean scores with placebo but with sodium hyaluronate there was a progressive reduction which was significant (p less than 0.01) throughout the period from 5 to 23 weeks. The difference between sodium hyaluronate and placebo was significant (p less than 0.05 to p less than 0.002) at Weeks 5, 11, 15, 19 and 23. 'Activities of daily living' were assessed using a standard scale. There were small improvements with both treatments, significant at some assessments and somewhat greater with sodium hyaluronate than placebo, but there were no statistically significant differences between the groups.(ABSTRACT TRUNCATED AT 250 WORDS)