Circulating antibodies to acetylcholine receptor protein (anti-AChR) were measured in the sera collected from 75 patients (53 women, 22 men, ages 9-83 year, 20 with a thymoma) with myasthenia gravis (MG) during 5-44 (mean 25) months. The clinical state of each patient was graded on a 6-point scale. Anti-AChR concentrations were measured by a radioimmunoassay with human antigen. We analysed the relation between the change in clinical state and the change in anti-AChR concentration in 155 periods (1-7, mean 2.1 per patient). The change in clinical state is given as the difference in score at the onset and at the end of this period. The change in anti-AChR concentration is expressed as the percentage of the original concentration at the onset of the period. The results were analysed in relation to the therapy and to the severity of the MG at the onset of each period. A strong correlation between a change in anti-AChR concentration and a change in clinical condition existed during treatment with prednisone or immunosuppression and in the period after thymectomy, while a weaker correlation was present in periods without immunosuppression. In only 3 patients did the changes in anti-AChR concentration precede the clinical change. No changes in anti-AChR concentrations were found if improvement was due to the effect of anticholinesterases or if deterioration was caused by infection or emotion. The serial measurement of anti-AChR may be a valuable method of following the basic trend of the MG in severely affected patients.