Reactive thrombocytosis (RT; thrombocyte count: > 450 x 109/l) is a condition in which an increase in platelet production, stimulated by cytokines in the bone marrow, is secondary to some condition or circumstance. Although RT does often occur in children, the risk of thromboembolic complications is negligible in this group if they have no other risk factors for thrombosis. In the absence of additional risk factors for thrombosis it seems as though RT in adults does not predispose them to thromboembolic complications either. Patients with RT caused by a non-myeloproliferative malignancy do have an increased risk of thromboembolic complications; antithrombotic prophylaxis might be effective in this group, but there is no scientific evidence for this. We advise a watch-and-wait approach in children and adults with RT who have no other risk factors for thromboembolism, even in patients with extreme thrombocytosis (thrombocyte count: > 1000 x 109/l). In patients who do have an increased risk of thromboembolic complications we advise tailoring prescription or non-prescription of antithrombotic prophylaxis to the individual patient.