Nine patients with severe myasthenia gravis, including ocular symptoms, who did not respond to anticholinesterase therapy were given prolonged courses of corticotrophin. In all patients except one there was an initial deterioration, sometimes necessitating assisted respiration, followed by a marked improvement which persisted for many weeks and occasionally months. It is concluded that corticotrophin should be continued for at least two weeks as the onset of the remission might be delayed. Repeated courses or small maintenance doses prolonged the remission. Fluid retention and hypokalaemia were common unwanted effects but necessitated stopping the treatment in only one patient.