We analysed the GATA binding protein 4 gene, or GATA4, along with the NK2 transcription factor related, locus 5 gene, or NKX2.5, to determine their genetic contribution to 104 sporadic patients in Indonesia with congenitally malformed hearts, 76 cases having atrial septal defect and 28 tetralogy of Fallot. We found only 1 novel mutation of GATA4 in those with atrial septal defects. Analysis of the genetic background of the parents of the patient showed for the first time that a new mutation of GATA4 can cause sporadic atrial septal defects. We failed to discover any other mutations of either the GATA4 or NKX2-5 genes, supporting the marked genetic heterogeneity of human congenital cardiac defects.