Forty-three patients with myasthenia gravis (MG) were studied to compare the sensitivity of repetitive nerve stimulation (RNS) test with single fiber electromyography (SFEMG) in the diagnosis of MG. SFEMG was abnormal in 81% (35 of 43 cases), while RNS test was abnormal in 53% (23 of 43 cases). In ocular MG, SFEMG showed a much higher positive rate (70%, 16 of 23 cases) than RNS test (22%, 5 of 23 cases). Both studies had similar abnormal rates in the generalized group. In patients studied with SFEMG in the EDC muscle, the clinical severity was closely correlated with jitter abnormality. We conclude that SFEMG is more sensitive than RNS test in the diagnosis of MG, especially in the ocular group.