1. A study has been made 10 years after the end of treatment of the 497 children who were admitted to the U.K./U.S. cooperative clinical trial of the relative merits of ACTH, cortisone, and aspirin in the treatment of acute rheumatic fever. 2. Three hundred and ninety-seven of the cases (79.9 per cent) were known to be alive at 10 years and the status of the heart was known for 347 of them. In addition, 23 (4.6 per cent) had died, 19 from rheumatic fever and rheumatic heart disease, and 77 (15.5 per cent) were untraced. The very low fatality rate is striking. 3. At the end of 10 years, there is no evidence that, on the treatment schedule used in this study, the prognosis has been influenced more by one treatment than another. This confirms the findings reported at 1 year and at 5 years. 4. The most important factor in determining the prevalance of rheumatic heart disease at the end of 10 years is the status of the heart at the time treatment was begun. For cases initially without carditis the prognosis was excellent, since in 94 per cent there was no residual heart disease. In cases initially with carditis but without pre-existing heart disease, the proportion without residual heart disease was 70 per cent for those with only a grade I apical systolic murmur, and 74 per cent for those with only a grade II or III apical systolic murmur. The proportion without heart disease decreased to 32 per cent for those initially with failure and/or pericarditis. In cases with pre-existing heart disease, the prognosis was poor. Forty per cent of those initially without pericarditis or failure and only 11 per cent of those with pericarditis and/or failure were without heart disease at 10 years. 5. Mitral stenosis, uncommon at 5 years, was definitely diagnosed in 18 of the 347 cases examined at 10 years and in 7 of the 19 deaths from rheumatic fever and rheumatic heart disease. The prevalence of this complication increased with the severity of the cardiac status at start of treatment and was greater in females than in males. 6. Retreated recurrences of rheumatic fever in cases without pre-existing heart disease worsened the prognosis but did not increase the prevalence of mitral stenosis. Patients surviving an initial attack of rheumatic fever without residual heart disease do develop heart disease following a retreated recurrence. The effect of initial cardiac status on, the recurrence rate could not be determined. 7. Sex affected the outcome at 10 years. Rheumatic fever in its milder grades had a worse prognosis, and mitral stenosis was more common in females than in males. 8. These results make clear that the status of the heart of the patients at the start of treatment, the rate of recurrence of acute rheumatic fever, and the sex of the subjects must all be taken closely into account in the evaluation of any treatment of acute rheumatic fever.