Abstract-- Background: Most congenital myasthenic syndromes are caused by defects in postsynaptic or synaptic basal lamina-associated proteins; congenital myasthenic syndromes (CMSs) associated with presynaptic defects are uncommon. Here, the authors describe clinical, electrophysiologic, and morphologic features of two novel and highly disabling CMSs, one determined by presynaptic and the other determined by combined presynaptic and postsynaptic defects. Methods: Microelectrode, single channel patch clamp, immunocytochemical, [¹²⁵I]-bungarotoxin binding, and quantitative electron microscopy studies of endplates were performed. Candidate genes were directly sequenced. Results: Patient 1, a 7-year-old boy, had severe myasthenic symptoms since infancy. Patient 2, a 48-year-old man, had delayed motor milestones and became progressively weaker after age 2 years. Both used wheelchairs and had a 30-50% EMG decrement on 2-Hz stimulation. Evoked quantal release was reduced to approximately 25% of normal in both. In Patient 2, the synaptic response to acetylcholine was further compromised by degeneration of the junctional folds with concomitant loss of the acetylcholine receptor (AChR). A search for mutations in components of the synaptic vesicle release complex and in other candidate proteins failed to identify the molecular basis of the two syndromes. Conclusions: Combined clinical, morphologic, and in vitro electrophysiologic findings define two novel congenital myasthenic syndromes. The molecular basis of these syndromes awaits discovery.