This study was undertaken in 39 patients with Fallot's tetralogy associated with one or more cardiovascular malformations. Some common anomalies, simple anatomical variations were excluded (right sided aortic arch, patent foramen ovale). Out of a total of 48 anomalies (6 children had 2, 3 or 4 associated malformations) there were 5 anomalous systemic venous drainages (including 4 supernumerary left superior vena cava), 8 secundum atrial septal defects (including one multiperforated defect), 2 muscular ventricular septal defects, 7 atrioventricular canals (5 in mongol children); 1 tricuspid hypoplasia, 1 absent left pulmonary artery, 1 mitral valve prolapse, 1 obstructive cardiomyopathy, 10 coronary anomalies (including 5 ectopic left anterior descending arteries arising from the right coronary artery), 8 patent ductus arteriosus (arising from the left subclavian in 4 cases of right aortic arch), 3 retro oesophageal subclavian arteries and one congenital subclavian steal syndrome. The incidence of associated malformations was 15 p. 100, but values of up to 30 p. 100 have been reported mainly in anatomical studies. The authors emphasise the diagnostic and therapeutic implications of these associated malformations. In most cases, especially Fallot's tetralogy with an endocardial cushion defect, 2D echocardiography proved to be an essential diagnostic tool. Catheter studies, however, remain necessary, especially for the detection of aortic and coronary anomalies: selective coronary angiography is sometimes required. Difficult surgical problems are mainly encountered when the associated anomaly has not been diagnosed preoperatively. Although the variations of systemic venous drainage are not of great importance, those of the aortic arch can influence the technique of palliative procedures. Muscular VSD must not be missed. Tricuspid hypoplasia may pose a complex surgical problem. Malformations of the left heart which are sometimes unrecognised, aggravate the situation considerably. The association of Fallot's tetralogy and atrioventricular canal can now be corrected under good conditions. Finally, variations in the coronary anatomy must be documented meticulously as they may contraindicate early complete repair.