In recent years, an increasing number of modified Fontan-operations has been performed in children younger than 4 years of age. The purpose of this study was to identify preoperative risk factors in this age group. From February 1990 until February 1993, we performed in our center a modified Fontan-operation using the technique of total cavopulmonary anastomosis (TCPA) in 37 consecutive patients (17 pts. < 4 years = group I, 20 pts. > 4 years = group II). Early postoperative mortality occurred in patients of group I only (n = 3 pts.). All of these patients had additional preoperative risk factors. Pulmonary vascular resistance (PVRI) > 2 U x m2 was a significant risk factor for the younger patients while pulmonary artery size alone (expressed as the McGoon-ratio or Nakata-index) could not be identified as a separate risk factor. Using two additional indices (McGoon-ratio/PVRI and Nakata-index/PVRI), we were able to identify patients with unfavorable postoperative hemodynamics as high-risk patients. In our experience, TCPA can be performed in patients younger than 4 years of age with a low mortality, if there are no additional preoperative risk factors. For high-risk patients we recommend either a bidirectional Glenn-anastomosis as a first step procedure or a TCPA with fenestration of the intraatrial tunnel-patch.