Glucosamine sulfate is a controversial osteoarthritis remedy that is presumed to stimulate articular cartilage glycosaminoglycan synthesis by increasing glucosamine concentrations in the joint space. However, this is not plausible because even large oral doses of the product have no effect on serum glucosamine concentrations. We propose instead that sulfate could mediate the clinical benefit attributed to this treatment. Sulfate is required for glycosaminoglycan synthesis, and unlike glucosamine, its serum level can be modified by dietary and other factors. In this study, we tested whether oral glucosamine sulfate increases serum sulfate concentrations and whether the sulfate concentration in the synovial fluid reflects that in the serum. The serum sulfate concentration of 7 normal subjects was 331 +/- 21 micromol/L before ingestion of 1.0 g glucosamine sulfate and 375 +/- 17 micromol/L 3 hours after (P <.05). Serum sulfate concentrations decreased from 325 +/- 19 to 290 +/- 19 micromol/L when the same dose of glucosamine sulfate was ingested with 1.0 g of the analgesic drug acetaminophen, which is largely metabolized by sulfation (P <.05). Unlike glucosamine sulfate, oral sodium sulfate did not significantly increase the serum sulfate concentration. Synovial fluid and serum sulfate concentrations were closely similar when measured in 15 patients undergoing diagnostic needle aspiration of a knee effusion (r =.99, slope =.97, P <.0001). These results do not prove that glucosamine sulfate improves osteoarthritis, but considered with other data, they do provide a plausible biochemical mechanism for its reported beneficial effects. This hypothesis is clinically relevant because it predicts that nonsulfate salts of glucosamine will be ineffective and that renal function, diet, and concurrent acetaminophen therapy could confound clinical trials of this therapy.