We sought to determine if a higher dose of heparin would reduce arterial complications in patients weighing 10 kg or less undergoing cardiac catheterization to investigate congenital heart disease. Sixty patients were given either 100 (group A) or 150 (group B) IU x kg(-1) of heparin in a double-blinded randomized manner. Initial arterial access was established using a 4F cannula in all patients. Mean activated clotting time measured 20 minutes following heparin administration was significantly lower in group A than in group B (199 versus 251 seconds). Only 3 out of 60 patients (5%) required treatment for loss of femoral pulse. The age, weight, activated clotting time, length of catheterization procedure, time taken to establish arterial access, and the duration of arterial cannulation were comparable between the groups. Weight under 4 kg, age under 1 month, and cannula size larger than 4F were identified as independent risk factors for the development of arterial complications. Arterial access using a 4F cannula is a safe procedure in children weighing 10 kg or less. The incidence of significant arterial complications is low, and they do not appear to be preventable by a higher dose of heparin.