Thrombotic complications of percutaneous arterial catheterization still remain a significant and serious problem in infants and children. Systemic heparinization has been recommended for prevention of these complications. The purpose of this study was to evaluate the effect of intraarterial injection of heparin in reducing thrombotic complications following percutaneous femoral artery catheterization. One hundred sixteen consecutive patients (ages four months to 20 years) studied by the Desilets-Hoffman modification of Seldinger's technique of femoral artery catheterization were randomly allocated to the control or heparin groups using a double-blind technique. At the completion of the catheterization, 0.1 mg/kg of placebo or heparin (1,000 units/ml) was injected into the common iliac artery prior to removal of the catheter and sheath. Segmental plethysmography was performed in both lower extremities prior to and after the catheterization, and a plethysmography index (PI) was calculated. The age and sex distribution, diagnoses, number, type, and site of previous catheterization, hemoglobin, platelet count, the amount of flush solution and the heparin contained therein, size of the catheter and sheath used, number of arterial punctures, and the length of the time in the artery were similar in the two groups (P greater than 0.1). Thrombin time and activated partial thromboplastin time were measured prior to the use of flush solution and prior to angiography, and these remained essentially unchanged in the two groups. The PI in the control group (97.5 +/- 320 was not significantly different (P greater than 0.1) from that of the heparin group (97.7 +/- 32). Similarly, the six to 24 month of postcatheterization plethysmography data show no differences (P greater than 0.1). The number of patients with reduced ipsilateral posterior tibial and dorsalis pedis pulses was also similar (P greater than 0.1). None of the patients in either group required thrombectomy. The low low incidence of arterial complications in our patients when compared with other studies may be related in part to the use of a sheath, which is not called for in original Seldinger technique. The data suggest that full-dose heparin administration does not significantly alter arterial complications following percutaneous femoral artery catheterization, especially in children over five years of age.