The increased survival of low birth weight infants means that more of these infants may be candidates for catheter interventions. There are few data on the results of cardiac catheterization in this group. This study aimed to analyze, retrospectively, cardiac catheterization of infants weighing < or =2.5 kg, with emphasis on the results of interventions. The complication rates of interventional and diagnostic procedures were compared. One hundred eleven catheterizations were performed in 107 patients between 1985 and 1998. Thirty-one procedures were interventional. Balloon atrial septostomy (n = 16), balloon pulmonary valvuloplasty (n = 10), balloon pulmonary angioplasty (n = 1), and coil occlusion of collateral vessels (n = 3) were all performed successfully. One infant (3%) died while undergoing myocardial biopsy. The reintervention rate for isolated pulmonary valve stenosis was 25% (2 of 8) at 1 month, 57% (4 of 7) at 6 months, and 71% (5 of 7) at 1 year. Complications were significantly more frequent during interventional (13 of 31, 42%) than during diagnostic (13 of 80, 16%) procedures. The most common complications during interventions were arrhythmias (3 of 31, 10%) and respiratory deterioration (3 of 31, 10%). Cardiac catheterization was technically feasible in all patients. Balloon pulmonary valvuloplasty and atrial septostomy provided good palliation in this patient group. The mortality of interventional procedures was low. The high incidence of respiratory complications suggests that low birth weight infants should undergo elective ventilation for interventional cardiac catheterization.