OBJECTIVE Miniaturisation of the extracorporeal circuit is a current trend in modern paediatric cardiac surgery. Many investigators stress that reduction of priming volume and artificial surface area of extracorporeal circulation could lead to clinical and economic benefits. The aim of this paper was to evaluate the costs of mini-circuit use in infants undergoing open heart surgery. METHODS We assessed post-operative course and cost of treatment in 60 infants undergoing open heart surgery. This group was prospectively randomised and divided into 2 equal subgroups: with miniaturised (group M) and conventional cardio pulmonary bypass circuits (group C). The study groups were clinically comparable. Surgical complications, duration of hospitalisation and cost of postoperative treatment were assessed in both groups. RESULTS Miniaturisation of the extracorporeal circuit led to a significant reduction of priming volume and artificial surface area (by 46.6% and 68.8% respectively, p=0.0000001). Post-operative cardio-respiratory insufficiency (2 vs. 8, p=0.038), and infection (3 vs. 9, p=0.049) occurred less often in children from group M. Hospital stay was significantly shorter in group M. Total cost of treatment was significantly lower in children from group M (median: 4361.4 vs. 6660.5 euro, p=0.037). CONCLUSIONS Miniaturisation of the extracorporeal circulation significantly improve post-operative outcome in infants undergoing open heart surgery. The mini-circuit significantly reduces cost of treatment in small children undergoing open heart surgery.