Consecutive patients (n = 114), who had single fiber electromyography of the frontalis muscles for symptoms suggestive of ocular myasthenia gravis, were followed up for a mean of 14 months (3-64 mos). At follow up, based on strict criteria, 23 patients were classified as having ocular myasthenia gravis, 8 patients were diagnosed as having mitochondrial myopathy or oculopharyngeal dystrophy, 18 patients were found to have other diseases and 65 patients remained without a definite diagnosis. The single fiber electromyography data of these patients were then reviewed. The patients with ocular myasthenia gravis had, on average, more than 7/20 single fiber pairs with jitter > 45 μs and mean jitter of 56 μs. The 8 patients with mitochondrial myopathy or oculopharyngeal dystrophy had an average of 5/20 single fiber pairs with jitter > 45 μs and a mean jittter of 52 μs and could not be separated from the group with ocular myasthenia gravis on the basis of the single fiber electromyography results. The 18 patients with definite other diagnosis had an average of less than 1/20 single fiber pair with jitter > 45 μs and a mean jitter of 25 μs. This group could be clearly separated from the group with ocular yasthenia gravis. We conclude that single fiber electromyelography is useful in the separation of ocular myasthenia gravis from other causes of oculomotor weakness except mitochondrial myopathy and oculopharyngeal dystrophy.