In order to clarify factors of relevance to the rapid clinical deterioration in HTLV-I-associated myelopathy (HAM), we analyzed clinical and laboratory parameters of 28 patients. Patients were divided into rapid (n = 14) and slow progression groups (n = 14) by severity of paraparesis in the 5th year after onset. Clinically, only young age at onset of the disease was significantly associated with rapid clinical deterioration. Among laboratory parameters, depressed skin reactions to dinitrochlorobenzene and PPD, a depressed lymphoproliferative responses and increased CSF IgG levels were significantly associated with rapid clinical deterioration. Serum and CSF anti-HTLV-I antibodies titers were not a relevant factor in the rapid clinical deterioration. The results suggest the implication of immunopathogenic mechanisms in HAM.