Priapism is a true urological emergency that is typified by a persistent and painful erection. High-risk groups include patients with hematological or coagulative disorders; for example, those with sickle cell disease, leukemia or glucose-6-phosphate dehydrogenase deficiency. The diagnosis for priapism must be made urgently using patient history, physical examination and blood gas findings on corporal aspiration. Emergency treatment is needed to avoid erectile dysfunction. However, in high-risk groups, prophylaxis must be encouraged. A number of prophylactic measures are emerging based on progress in the understanding of the pathophysiology of priapism in these particular patients. In this Review, priapism as it relates to hematological disorders is discussed, focusing on treatment and prophylaxis.