Caspofungin, aechinocandin, is a relatively new lipophilic antifungal drug. Little is known concerning the pharmacokinetics of caspofungin in children. Extracorporeal membrane oxygenation (ECMO) allows prolonged cardiopulmonary support in patients with life-threatening respiratory or cardiac failure. Pharmacokinetics may be altered by ECMO. We describe the case of a paediatric patient on ECMO with severe pneumonia and sepsis, who had subtherapeutic exposure of caspofungin despite normal to high dosages of caspofungin. Therapeutic drug monitoring is warranted.