Two children, now 5 1/2 and 6 years of age, presented as neonates with hypotonia, multiple joint contractures, ptosis, extraocular weakness, bulbar symptoms, and respiratory distress. Fluctuations and episodic exacerbations of weakness necessitated respiratory support. Both children are developmentally delayed and cannot walk independently, although one child underwent bilateral tenotomies. Biochemical investigations and electromyography, including slow-rate, repetitive nerve stimulation, were normal. Acetylcholine receptor antibodies in serum were absent. Single-fiber electromyography with axonal stimulation revealed prolonged mean jitter in the tibialis anterior and extensor digitorum muscles, with more than 2 abnormal individual jitter values in each muscle. Muscle biopsy demonstrated normal pattern and morphology of muscle fibers; immunohistochemical staining for cholinesterase was positive. Electron microscopy revealed abnormalities in motor endplates: atrophy, flattening of primary synaptic clefts, and paucity of side branches. These findings represent one of the postsynaptic abnormalities (i.e., acetylcholine receptor deficiency or paucity of synaptic folds). Both children improved clinically on pyridostigmine therapy. Arthrogryposis congenital multiplex due to congenital myasthenic syndrome, as diagnosed in our patients, has been reported once before. The diagnosis can be established by clinical history, neurologic examination, and electrophysiologic and pathologic findings. Clinical improvement can be achieved with high-dose anticholinesterase therapy.