Here we present the case of a patient with diagnosis of chronic lymphocytic leukaemia (CLL) on routine protocol biopsy 3 months following kidney transplantation. Genetic analysis confirmed the origin of the malignancy, being the recipient. Differential diagnosis with post-transplant lymphoproliferative disorder (PTLD) is extremely important in order to avoid unnecessary devastating treatment. This case is challenging, both in terms of making the correct diagnosis and in terms of optimal treatment. The case underscores that it is extremely important to distinguish between a pre-existing lymphoma diagnosis after transplantation and a true PTLD as the treatment options of both are very divergent.