Human T-cell lymphotropic virus type-I (HTLV-I)-associated myelopathy/tropical spastic paraparesis (HAM/TSP), the main neurological manifestation of HTLV-I, is a chronic inflammatory disease. Viral-host interaction and host genetics are two important contributors to the development of the HAM/TSP. This study was conducted to measure the serum level of tumor necrosis factor-alpha-like weak inducer of apoptosis (TWEAK) by ELISA method in three groups of participants including 34 HAM/TSP patients (HAM/TSP), 35 asymptomatic HTLV-1 carriers (ACs), and 20 healthy controls (HCs). Also, the titer of the proviral load in two groups of HAM/TSP and ACs was assessed by the real-time polymerase chain reaction (PCR). The statistical results showed that, there is no significant difference between the three groups in TWEAK cytokine level (p = 0.667). Also, there was no significant difference in proviral load titer between groups of HAM/TSP and ACs (p = 0.08). Furthermore, no significant difference was observed between proviral load and TWEAK cytokine concentration between groups of HAM/TSP and ACs. Our findings showed that despite the inflammatory nature of HAM/TSP disease, the expression level of TWEAK in HAM/TSP patients is not significantly different from the groups of ACs and HCs. Therefore, the involvement of other factors in causing HAM/TSP is not unexpected.