Concentrations of circulating cartilage-derived keratan sulfate (KS) are significantly higher in patients with osteoarthritis (OA) than in adults without cartilage disease. The increases in serum KS levels in the OA group are likely the result, at least in part, of cartilage degradation in affected joints. If the appearance of elevated levels of serum KS do indeed correlate with the extent of cartilage erosion or destruction in individuals with OA, measurements of serum KS levels will prove extremely useful in the assessment and diagnosis of this joint disease.