The acute complications of therapeutic cardiac catheterization for congenital heart disease as performed currently in a small unit were reviewed. In recent years, there has been a significant increase in the number of lesions thought amenable to catheter therapy. Only a few reports, however, have addressed the overall incidence of acute complications of therapeutic cardiac catheterization, all representing the experience of centres performing moderate-to-large numbers of procedures. A retrospective review was performed of 425 therapeutic catheter procedures performed at our institution between May 1993 and November 1997. Acute complications were retrieved from the database. This included all adverse events that were clinically recognized at the time of or within 2 weeks after the procedure and which, to the best of the authors' clinical judgement, were related to the catheterization and not part of the natural history of the child's illness. All patients were observed overnight following the procedure, and stayed in hospital if a complication developed. There were 49 acute complications (11.5%), of which 43 (10.1%) were deemed minor and 6 (1.4%) were considered major. The rate was low in patients with valvar pulmonary stenosis, including three neonates (3/45, 6.7%), for those undergoing angioplasty of native co-arctation (1/15, 6.7%) and pulmonary arteries (2/21, 7.4%); and for coil embolization of systemic to pulmonary collateral arteries (1/16, 6.3%). The rate was high in patients with valvar aortic stenosis, including 12 neonates (9/37, 24.3%), and for angioplasty of re-coarctation (4/23, 21.7%). There were more overall complications in neonates (25.6%) than in older patients (10.1%) (p < 0.01). Two patients died (0.5%), but no patient required emergency surgical intervention. In spite of the introduction of many new therapeutic modalities with greater intrinsic risk, and the fact that patients with more complex lesions and who are more acutely ill are being treated, the overall rate of complications remains relatively low. This probably reflects improvements in pericatheterization medical management, in selection of patients, in procedural techniques, and in the experience of operators.