Nitric oxide (NO) has an important role in the inflammatory arthropathies. This study investigated NO levels in the synovial fluid and plasma of patients with primary osteoarthritis (OA) of the knee. Twenty-seven cases with primary knee OA and 13 controls were recruited for the study. Nitrate/nitrite levels of synovial fluid and plasma were measured by Griess reaction, and interleukin-1beta (IL-1beta) levels were measured quantitatively by a sandwich immunoassay technique. We found a significant increase in the synovial fluid nitrate/nitrite levels in cases with primary OA of the knee compared to controls (50.26+/-23.63 mug/l vs 32.49+/-10.05 mug/l, p=0.002) as well as increased plasma nitrate/nitrite levels (57.06+/-23.32 mug/l vs 39.98+/-16.36 mug/l, p=0.012). There was no difference in plasma and synovial fluid IL-1beta concentrations between the study and control groups. These results may be considered as supporting evidence that NO might be one of the factors responsible for cartilage destruction in primary osteoarthritis of the knee.