The author presents a case of a 52-year old woman who suffered from dermatitis seborrhoica. She was treated with chloroquine. During the third month of treatment with chloroquine the symptoms of myasthenia appeared and led to hospitalization. The neurological and additional examinations confirmed the diagnosis. Four days after withdrawal of chloroquine treatment the myasthenic symptoms began to disappear and they finally ceased after two weeks. At this all other drugs were also withdrawn. The control examination after two months showed normal neurological status.