The clinical condition and the serum levels of antibodies to acetylcholine receptor protein were followed in 30 patients with myasthenia gravis before and in a period varying from 2 to 4 (mean 3) yr after thymectomy. Twenty-five patients improved in the 2 yr following thymectomy. A highly significant correlation was found between the change in clinical condition and the change in antibody levels. Only 3 patients improved without a fall of antibody level. Prethymectomy antibody levels were positively correlated with the severity of the clinical condition and with the degree of thymus hypertrophy.