BACKGROUND The relationship between human T-lymphotropic virus type 1 (HTLV-1) and mycosis fungoides, a type of cutaneous T-cell lymphoma, remains obscure. While HTLV-1 infection has been closely associated with adult T-cell leukemia/lymphoma (ATLL), there is limited data on its relationship to other cancers derived from the immune system, such as mycosis fungoides. Here, we aim to present a retrospective case series of Peruvian patients with HTLV-1 infection who were also diagnosed with mycosis fungoides. METHOD We conducted a secondary data analysis to investigate the association of mycosis fungoides and HTLV-1 in patients at Hospital Nacional Cayetano Heredia and the Instituto de Medicina Tropical Alexander von Humboldt (IMTAvH) in Lima, Peru. Demographic information, clinical records, laboratory reports, and biopsy reports (e.g., skin, bone marrow, lymph nodes) were reviewed. Current status was checked using national registers. We used descriptive statistical methods to obtain the results. RESULTS We describe 26 patients with mycosis fungoides and HTLV-1. Of these, 9 (35%) were male and 17 (65%) female. Regarding ethnicity, 12 (46%) were mixed race, 11 (42%) were Andean, 1 (3%) was white, 1 (3%) Asian, and 1 (3%) unspecified. The median age at HTLV-1 diagnosis was 63 years (IQR 53–73.5), and the median age at mycosis fungoides diagnosis was 65 years (IQR 54–75). Eight (30%) were alive at last follow-up, 16 (61%) were deceased, and 2 were lost to follow-up. Regarding other HTLV-1–associated manifestations, 6 patients (23%) had infective dermatitis, 1 (3%) had scabies, and 1 (3%) had HTLV-1–associated myelopathy/tropical spastic paraparesis (HAM/TSP). CONCLUSION Through this data we describe a possible association between HTLV-1 and cutaneous T-cell lymphoma/mycosis fungoides. This is a poorly recognized association; to our knowledge, there are not many reports of these two entities. Further studies should be conducted to clarify the relationship between them. Additionally, we suggest that in countries with high prevalence of HTLV-1, patients with newly diagnosed mycosis fungoides should be assessed for HTLV-1 as well.