In Sweden, which has a population of 8.9 million people, pediatric heart surgery was previously performed in four cities. After a long, difficult process, centralization of pediatric heart surgery to two centers was achieved in 1993. The overall 30-day mortality for open-heart surgery on infants and children of 9.5% before the centralization (1988-1991) was reduced to 1.9% in 1995-1997. A causal relationship between the mortality rates before and after the centralization is impossible to prove. Heart surgery was concentrated to the two centers with the lowest surgical mortality, and the reduction in surgical mortality was observed over a short period of time which makes it likely that the centralization of the surgical activity promoted the improved results. During the later time period the amount of more complex surgery was clearly increased compared to that performed previously.