The purpose of this prospective study is to analyse the immediate effect of Rashkind's atrioseptostomy on systemic saturation in transposition of the great arteries (TGA). Thirteen neonates and infants (10 males and 3 females) with TCiA underwent balloon atrial septostomy (BAS) at a median age of 20 days (range 2 and 60 days). The mean atrial septal defect diameter after BAS was 6.5 +/- 1.1 mm. The right ventricular saturation increased from 37 +/- 17% to 67 +/- 13% (p < 0.001). There was no correlation between the atrial septal defect diameter and the increase of systemic saturation after BAS.