We report a 47-year-old Japanese woman with HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) combined with acute type adult T-cell leukemia (ATL). The susceptibility for HAM/TSP and acute type of ATL is hitherto explained by human leukocyte antigen (HLA) haplotype-linked immune responsiveness to HTLV-I. This patient's HLA (A24Cw1B54DR4DQ4/A24Cw3B51DR8DQ1) included a HAM-associated HLA haplotype. This suggests that HAM patients with HAM-associated HLA haplotype can also develop the acute type of ATL.