We have assessed nitric oxide (NO) products, nitrites and nitrates in the sera of 80 controls and 14 patients with multiple sclerosis (MS) in different stages of clinical activity: seven with clinical remission (RR) (clinically inactive) and seven with chronic progressive (CP) disease (four active and three inactive). Overall, the levels of nitrites, nitrates and total NO products were significantly lower (p < 0.001) in MS patients than in controls. However, this decrease was significant only in patients with RR (P < 0.0001) and inactive CP (P < 0.0005). Likewise, patients with RR and inactive CP patients had significantly (P < 0.00 1 and P < 0.05 respectively) lower levels of NO products (16.92 ± 3.1 μmol/l and 18 ± 3.17 μmol/l respectively) as compared to active CP patients (24.1 ± 2.81 μmol/l). Serum NO levels may vary with the stage and activity of the disease. Future studies should ascertain the importance of serum NO in MS.