The human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus first identified 30 years ago as the first human oncogenic retrovirus. While not widespread, HTLV-1 is present in various regions worldwide, with high endemic areas in Southwestern Japan, sub-Saharan Africa, South America, the Caribbean, and parts of the Middle East and Australo-Melanesia. The distribution is likely due to a founder effect in some populations with persistent high transmission rates. HTLV-1 prevalence increases with age, especially among women in high-endemic areas. Transmission occurs through mother-to-child, sexual contact, and contaminated blood products. Previous estimates suggested 10–20 million infected individuals, but recent data indicate 5–10 million. However, reliable data are limited in many regions, including China, India, the Maghreb, and East Africa, making the true global number uncertain. HTLV-1 prevalence varies by region, with high rates in specific clusters. In Europe, HTLV-1 is mainly found in Romania and in immigrant populations from high-endemic areas. In Africa, HTLV-1 is widespread, with varying prevalence rates across different countries. In the Americas, HTLV-1 is endemic in parts of South America, particularly in Colombia, Peru, and French Guiana, where specific populations are highly infected. In North America, HTLV-1 is rare, with low prevalence among blood donors. HTLV-1 molecular epidemiology shows distinct subtypes, with the Cosmopolitan subtype A being the most widespread. The global distribution and prevalence of HTLV-1 remain challenging to estimate due to limited data and the uneven distribution of the virus. The article provides an overview of the current understanding of HTLV-1 epidemiology, its distribution, and the challenges in estimating its global prevalence.The human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus first identified 30 years ago as the first human oncogenic retrovirus. While not widespread, HTLV-1 is present in various regions worldwide, with high endemic areas in Southwestern Japan, sub-Saharan Africa, South America, the Caribbean, and parts of the Middle East and Australo-Melanesia. The distribution is likely due to a founder effect in some populations with persistent high transmission rates. HTLV-1 prevalence increases with age, especially among women in high-endemic areas. Transmission occurs through mother-to-child, sexual contact, and contaminated blood products. Previous estimates suggested 10–20 million infected individuals, but recent data indicate 5–10 million. However, reliable data are limited in many regions, including China, India, the Maghreb, and East Africa, making the true global number uncertain. HTLV-1 prevalence varies by region, with high rates in specific clusters. In Europe, HTLV-1 is mainly found in Romania and in immigrant populations from high-endemic areas. In Africa, HTLV-1 is widespread, with varying prevalence rates across different countries. In the Americas, HTLV-1 is endemic in parts of South America, particularly in Colombia, Peru, and French Guiana, where specific populations are highly infected. In North America, HTLV-1 is rare, with low prevalence among blood donors. HTLV-1 molecular epidemiology shows distinct subtypes, with the Cosmopolitan subtype A being the most widespread. The global distribution and prevalence of HTLV-1 remain challenging to estimate due to limited data and the uneven distribution of the virus. The article provides an overview of the current understanding of HTLV-1 epidemiology, its distribution, and the challenges in estimating its global prevalence.