In recent years, there has been an increase in the use of tumour necrosis factor (TNF) inhibitors as treatment for several inflammatory conditions. However, the question of whether TNF inhibitors increase the risk of malignancies (including lymphoma and leukaemia) in these diseases remains controversial. Despite this concern, anti-TNF therapy is being used experimentally in the management of haematological patients with risk of leukaemic transformation such as myeloproliferative neoplasms. We report here the first ever reported case of blastic transformation in a patient with myelofibrosis under etanercept treatment for a severe hidradenitis suppurativa. Although etanercept provided a sustained partial response of the skin disease, the patient developed an acute myeloid leukaemia after 27 months on exclusively etanercept therapy. According to the Dynamic International Prognostic Scoring System-plus score, the patient had a low risk for leukaemic transformation. We discuss here the potential of TNF inhibitors to increase the already elevated risk of leukaemic transformation of these haematological diseases.