Simon Rule, MD from Derriford Hospital, Plymouth, UK provides an overview of the side-effects of ibrutinib, based on data presented at the American Society of Hematology (ASH) 2015 Annual Meeting, on the RAY (MCL3001) study of ibrutinib vs temsirolimus in relapsed/refractory mantle cell lymphoma. In mantle cell lymphoma, the side-effects that cause the most problems are bleeding and atrial fibrillation, for which the frequency was similar in both arms of the RAY study.