Two patients are described with a myasthenic syndrome that presented in early adult life. One patient had 2 asymptomatic first degree relatives with similar electrophysiological findings. Both patients had abnormal fatiguability, arm weakness being prominent; neither of them responded to anticholinesterase medication. An abnormal decrement at 3 Hz stimulation was present, and a single stimulus evoked a repetitive response. Electrophysiological studies on biopsied intercostal muscle showed miniature endplate potentials of normal amplitudes but with prolonged rise and decay times. Anticholinesterase staining (Case 1) was not reduced, and showed elongation of some endplates. Ultrastructural studies (Case 2) showed degeneration of junctional folds and diffusely thickened endplate basal lamina. Calcium deposits were not observed and myopathic changes were slight. The findings are consistent with a prolonged open time of the ACh-induced ion channel.