A study was made of two renal biopsy specimens obtained from a 22-year-old woman with Bartter's syndrome, the first to substantiate the diagnosis, the second 2 years later after the treatment with spironolactone. The first renal biopsy revealed remarkable hyperplasia and hypertrophy of the juxtaglomerular apparatus. The numerous proliferating cells were characterized by abundant lysosomal granules and dilated endoplasmic reticulum. The macula densa revealed slight proliferation, with narrow intercellular space. The ultrastructural picture of the second biopsy specimen showed increased lipofuscin-like granules in the juxtaglomerular cells, with epitheloid cells of the macula densa showing degeneration and irregular dilatation of the intercellular spaces. However, the light microscopic finding was compatible with that of the first. These findings suggest that the treatment was inadequate although clinical features or biochemical laboratory data were improved.