The two cases reported were studied in the service of Dr. T. H. Weisenburg at the Philadelphia Infirmary for Nervous Diseases. The chief interest rested in the fact that the patients were sisters, with symptoms and signs that seemed to justify the diagnosis of myasthenia gravis. The rarity of the occurrence in sisters is easily confirmed by a review of the literature of the condition. REPORT OF CASES Case 1. —The younger sister, Anna F., showed the more pronounced symptoms, and the onset was earlier in her case. She was 17 years of age at the time of admission, June 18, 1926, but looked considerably older—probably about 21. The chief complaint was a general tired feeling, with general weakness in the muscles all over the body and inability to keep the eyelids open. In the family history of the two patients, there is one significant fact: the father is