Neurologic complications are a known and at times tragic consequence of cardiac catheterization. During a four-year period, 1,362 procedures were performed at our institution on children less than 15 years of age. Eighteen children, without prior nervous system disease, developed neurologic sequelae within 24 hours of cardiac catheterization (1.3%). Three presented with seizures only, ten with neurologic deficits, and five with both focal seizures and stroke. Computerized tomography scans (16 patients) demonstrated areas of nonhemorrhagic infarction in all but one. The 18 patients were studied retrospectively for possible factors contributing to the neurologic complications. They were matched to controls by age, cardiac lesion, and catheterization technique. Data examined included catheterization duration and approach, contrast agent, precatheterization urine specific gravity, pre- and postcatheterization hematocrit, blood loss, and intracatheterization events. Catheter and catheter sheath clots were noted in both patient groups. The neurologic complication group had 7/18 intracatheterization neurologic events versus only one in the controls and the change in hematocrit was greater in the complication group (t = 2.89, p less than 0.01). Due to this study's results and to prevent possible thromboembolic events, a heparinization protocol was instituted and patients were prospectively observed for neurologic complications. No neurologic complications have occurred over three years since the institution of this protocol.