Multiple sclerosis (MS) and HTLV-I associated myelopathy/tropical spastic paraparesis (HAM/TSP) can overlap in their clinical features and thereby cause difficulties for clinicians in relation to diagnosis and therapy. However, epidemiological biochemical, immunological, virological and radiological studies point to a number of significant differences. Recent comparative neurophysiological data, induding blink reflex studies, obtained in these disorders, is briefly reviewed here and provides additional evidence of difference. The abnormal blink reflex in patients with MS consist of prolonged latencies and absences of R1 and R2 responses and are mainly due to demyelinating lesions around the pons. In contrast, in HAM/TSP the blink reflex abnormalities frequently include an unusual early response, R/k, which is probably a consequence of interneuronal hyperexcitability around the brainstem. Thus these findings provide further support for our contention that HAM/TSP and multiple sclerosis are distinctly different both as clinical entities and in their underlying pathomechanisms.