Ibrutinib is a small molecule that has been recently developped for the treatment of B cell malignancies. Common side effects are diarrhoea, nausea, fatigue, infections, neutropenia and thrombocytopenia. Here we report the first case of Hepatitis B virus reactivation in a 80 years old chronic lymphocytic leukaemia patient receiving ibrutinib, suggesting that such treatment must be associated with HBV screening.