We studied the distribution of CD44-highly positive cells in relation to its ligand hyaluronic acid (HA) and basement membrane in hyperplastic thymuses from myasthenia gravis (MG) patients. CD44-highly positive cells were markedly increased in the hyperplastic MG thymuses as compared with control thymuses. They were present in subcapsular and cortical areas, sometimes extending to the medullary areas. The distribution of CD44-highly positive cells in the MG thymus was close to the sites with strong HA staining at blood vessel walls and extraparenchymal connective tissue. In the hyperplastic MG thymuses, the basement membrane surrounding extralobular blood vessels and the lobules was perforated or interrupted. These changes may allow the CD44-highly positive cells to penetrate into the hyperplastic MG thymuses.