Microsatellite instability (MSI) represents a replication error resulting from the dysfunction of mismatch repair gene products. In this study, MSI was analyzed in 18 patients with various subtypes of adult T cell leukemia/lymphoma (ATL/L). Using six different microsatellite loci, we defined MSI as positive when replication errors were observed in at least two loci. The MSI was positive in four cases (22.2%)with acute type ATL, who tended to show more prognostically unfavorable factors and shorter overall survival. These results suggest that genomic instability may be associated with tumor progression rather than the development of ATL/L itself. In addition, the presence of the MSI at initial presentation could appear to warrant consideration as an additional prognostically unfavorable factor.