OBJECTIVES: In the majority of cases, resistance to activated protein C is caused by the point mutation Arg⁵⁰⁶ to Gln in the factor V gene and has emerged as the most important hereditary cause of thromboembolism. METHODS: To determine to what extent resistance to activated protein C was present in children with thromboembolism and underlying cardiac disease, its occurrence was retrospectively investigated. By using a method based on activated partial thromboplastin time, with DNA technique derived from the polymerase chain reaction, we investigated nine children with underlying cardiac disease in whom thromboembolism had previously occurred. RESULTS: Heterozygous Arg⁵⁰⁶-to-Gln mutation in the factor V gene was diagnosed in five of the nine children investigated. In addition, protein C type I deficiency w as found in three patients, and two of the nine children showed increased lipoprotein (a) plasma values. Risk factors were present in all children with symptoms. CONCLUSIONS: These data indicate that deficiencies in the protein C anticoagulant pathway are likely to play an important role in the early manifestation of thromboembolism in children with underlying cardiac disease. (J Pediatr 1996;129:677-9)