A 21-year-old man presented to the Lausanne University Hospital (CHUV) in August, 2020, with reddened skin lesions on the forehead and upper torso, clinically suggestive of skin lymphoma. Skin biopsy, bone marrow aspirate and trephine biopsy, and PET-CT with 18F-fluorodeoxyglucose (18F-FDG) subsequently led to a diagnosis of T-cell acute lymphoblastic leukaemia, of mature T-cell phenotype, with skin infiltration. After diagnosis, the patient was enrolled in the GRAAL-2014/T clinical trial (NCT02619630).