Glucosamine sulfate is a drug used for the treatment of osteoarthritis (OA), based on its pharmacological and metabolic activities on the cartilage and chondrocytes, complemented by mild anti-inflammatory properties and a favorable pharmacokinetic profile. The aim of this study was to define the activity and safety of glucosamine sulfate on the symptoms of patients with OA, using a multicenter, randomized, placebo-controlled, double-blind, parallel-group study design. The study included 252 outpatients with OA of the knee (Lequesne's criteria), radiological stage between I and III, and Lequesne's severity index of at least 4 points and symptoms for at least 6 months. Patients were treated with either placebo or oral glucosamine sulfate 500 mg t.i.d. for 4 weeks, with weekly, with weekly clinic visits. Responders to treatment were defined as patients with a reduction of at least 3 points in the Lequesne's index with a positive overall assessment by the investigator. The Lequesne's index was 10.6 +/- 0.45 S.E.M. points in both groups at the start of the study. This decreased to 7.45 +/- 0.5 points in the treatment group (average 3.2) and 8.4 +/- 0.4 points in the placebo group (average 2.2) (P < 0.05, Student's t-test). The responder rate in the evaluable patients was 55% with glucosamine (N = 120) vs 38% with placebo (N = 121). These proportions were 52% vs 37% in an intention-to-treat analysis (P = 0.014 and 0.016, respectively; Fisher's Exact Test). The medications were well tolerated throughout the study, with no difference between the glucosamine and placebo treated groups. It is concluded that glucosamine sulfate may be a safe and effective symptomatic Slow Acting Drug for OA.