Serial acetylcholine receptor antibody (AChR-ab) titers have been followed in 32 patients with myasthenia gravis (MG) for an average of 43 months (range 12-81 months). Seventeen patients became asymptomatic or markedly improved during the study, 12 of whom showed AChR-ab decreases of greater than 50% sustained for 12 months. Only three of 15 patients showing no substantial clinical improvement had AChR-ab decreases greater than 50%. Steroids were more often associated with AChR-ab suppression than was thymectomy. The differences in AChR-ab suppressibility among identically treated patients were not predictable by any clinical characteristics studied. The study indicates a strong relationship between clinical course and AChR-ab in the individual patient when sustained improvement over 12 months and AChR-ab decreases greater than 50% are examined.