RESUMEN
BACKGROUND: Myeloproliferative neoplasms are a group of diseases with diverse biological and clinical characteristics. As a provisional separate entity, myeloid/lymphoid neoplasms with eosinophilia and genetic rearrangement have been described, which may present an initial clinical behavior of myeloproliferation and be characterized by varied genetic rearrangements. One of these entities is associated with FGFR1 rearrangements, characterized by its low prevalence and few treatment options. CASE PRESENTATION: We present the case of a 53-year-old Mestizo male patient of Hispanic origin who initially presented weight loss and fatigue, with a complete blood count showing leukocytosis and eosinophilia, with an initial diagnosis of nonspecific myeloproliferative disorder. In a next-generation sequencing study, BCR::FGFR1 rearrangement was documented, a diagnosis of myeloid/lymphoid neoplasia with eosinophilia and BCR::FGFR1 rearrangement was made, and hydroxyurea therapy was initiated. Subsequently, transformation to cortical T-lymphoblastic leukemia/lymphoma and erythroid precursors was documented, requiring management with chemotherapy. CONCLUSIONS: Myeloid/lymphoid neoplasms with eosinophilia and genetic rearrangements constitute a group of deeply heterogeneous diseases with variable clinical and diagnostic characteristics and whose treatment is not clearly defined.