Pathological changes, as determined by light or electron miscroscopy or both of percutaneous renal biopsy specimens, were present in 11 of 16 children and six of ten adults with acute rheumatic fever. Most of the patients with renal changes had clinical and laboratory evidence suggestive of renal disease. The highest index of suspicion was in patients with proteinuria or carditis. This was not universally so, however, and in two adults and two children no such evidence was noted. Renal lesions in acute rheumatic fever are in several ways distinct from those found in acute glomerulonephritis.