Priapism is defined as prolonged and persistent erection of the penis without sexual stimulation. It is associated with excessive hyperleukocytosis (e.g. in acute or chronic leukaemia); however, this complication is rarely seen in the pediatric population. We report a 12-year-old boy suffering from acute leukaemia presenting with, at first intermittent, but increasingly persistent erection. Doppler US revealed signs of high-flow priapism. MRI excluded intrapelvic tumour masses, and three-dimensional contrast-enhanced MR angiography could not demonstrate an arteriovenous fistula or thrombosis. Cavernosal blood-gas measurement was in agreement with high-flow priapism. On the basis of the imaging findings, invasive therapeutic management was avoided in our patient with a successful outcome.