Subjects with clinically confirmed LEMS or CM will receive 3,4-diaminopyridine (3,4 DAP) by mouth in slowly increasing doses. Treatment will begin with 5-10 mg three times a day. A common final dosage is 15-20 mg four or five times a day, as clinically needed, and if tolerated. The upper limit is a total of 100 mg/day. Subjects will be monitored for strength and side effects via routine clinic visits at intervals of one month for the first three months, then every three months for the first year, and at least every six months thereafter. Treatment will be continued indefinitely if a good clinical response is achieved and side effects are tolerable.