Clinical and electrophysiological findings in 16 consecutive cases of HTLV-I associated myelopathy are reported. Symmetric spastic paraparesis associated with only mild hyperreflexia in the arms, clinical and electrophysiological evidence of moderate posterior column involvement at the thoracic level, urinary frequency and urgency associated with the detrusor-urethral sphincter dyssynergia but without systemic autonomic involvement, and absence of the segmental grey matter involvement, all suggest the presence of diffuse lesions of the white matter predominantly involving the thoracic cord. We conclude that HTLV-I associated myelopathy is a chronic diffuse leukomyelitis predominantly involving the thoracic cord.