The authors report the diagnostic difficulties experienced with two siblings presenting with recurrent apnoeic attacks caused by the rare condition, familial infantile myasthenia. Standard repetitive nerve stimulation studies were normal in both cases, while changes typical of a subacute neurogenic condition were noted on concentric needle electromyography in one. A 14-day course of pyridostigmine did not elicit any clinical improvement in the elder sibling, in whom tracheomalacia was diagnosed by chest fluoroscopy and bronchoscopy. This infant died at the age of 11 months with hypoxic brain-damage after a severe apnoeic episode. For the second sibling, a positive decremental response was obtained in the hypothenar muscles only after two minutes continuous 10 Hz stimulation of the ulnar nerve. This child has responded well to treatment with pyridostigmine.