BACKGROUND Myasthenia gravis lacks a diagnostic gold standard, so diagnosis is supported by the findings of several tests. Conversion from ocular myasthenia gravis (OMG) to a more widespread disease is frequently early. AIMS. To describe and compare the clinical data and findings of the diagnostic tests of patients with OMG and those with a different diagnosis, and to know the conversion rate from OMG to generalized myasthenia. METHODS Descriptive and analytic retrospective study of 44 patients referred for neurophysiological testing through 4 years because of suspected OMG. RESULTS 12 patients (27%) were diagnosed as having OMG. Evolution time prior to diagnosis tended to be shorter in OMG patients. Isolated ptosis or combined with diplopia was more frequent in OMG, while isolated diplopia was so in other diagnoses (<I>p</I> = 0.003). No thymoma was found. SFEMG jitter of facial muscles was abnormal, including blocking, in all OMG patients (8/8) and normal in the rest (30/30). Edrophonium test was positive in all OMG patients (7/7) and doubtful in another one (1/7). Anti-AChR titers were initially positive in 10/11 OMG patients and 0/17 with other diagnoses. Aponeurotic ptosis and strabismus were the most frequent non-myasthenic etiologies. OMG showed an early generalization in two patients who developed dysarthria. CONCLUSIONS The low ratio of diagnostic confirmation suggests that in the face of ptosis or diplopia diagnostic testing with a high sensibility for OMG is favoured. Jitter showed the best initial diagnostic performance.