Human T-cell leukemia virus type 1 (HTLV-1) has been identified as a cause of adult T-cell leukemia-lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Despite several HTLV-1 endemic areas being identified, comprehensive investigations have yet to be conducted in all the regions of the world. This review aims to summarize the current reports on HTLV-1. As vertical transmission is known to be a risk factor for ATL development, prevention strategies have been initiated in Japan, and these efforts may be related to the decrease in the estimated number of HTLV-1 carriers in Japan. In numerous HTLV-1 endemic regions, the prevalence of HTLV-1 increases with age, which may be attributed to horizontal infection. However, the incidence of HTLV-1 infection appears to be high among adolescents and young adults in Japan, especially in non-endemic areas. The clinical significance of HTLV-1 infections, other than ATL and HAM/TSP, has recently been documented. Consequently, it is imperative to develop treatment strategies for HTLV-1 infections, including measures to prevent horizontal infections.