Recombinant hirudins have a definite role in the treatment of patients with heparin-induced thrombocytopenia (HIT). The most important adverse effects are haemorrhages and the induction of antihirudin antibodies. Major haemorrhages were not significantly increased in patients with HIT compared with a historical control group, but prospective data comparing hirudin and heparinoids such as danaparoid are lacking. The definition of the optimal method for monitoring and the availability of an antidote for hirudin would probably increase safety with this drug. To date, haemofiltration using high-flux filter systems is the only way to remove an overdosage of hirudin from the circulation. In patients with renal impairment requiring hirudin treatment, it therefore seems safer to start with a low dose that is subsequently adjusted according to the activated partial prothromboplastin time or ecarin clotting time.Even in special circumstances, such as cardiopulmonary bypass or dialysis, hirudins can be applied successfully if care is taken to monitor their effects meticulously.There are many other indications in which hirudins have shown feasibility (e.g. acute coronary syndromes) but available data preclude definite conclusions.