Recently it has been shown that in patients with transposition of the great arteries the isometric relaxation time of the left ventricle could be negative in the presence of normal pulmonary artery pressure. In order to find an explanation for this apparently paradoxical situation, it was decided to evaluate the importance of the delay of closure of the pulmonary valve in 15 patients with transposition of the great arteries. This delay is called the hang-out time. The hang-out time was found to correlate inversely with the isometric relaxation time as well as with the pulmonary artery systolic (r = -0 . 70) and mean pressures. A weaker correlation was found between the isometric relaxation time and the time of mitral valve opening. These results show that in transposition of the great arteries with normal pulmonary artery pressure, the pulmonary valve has a prolonged hang-out time, to the extent that it frequently closes after the opening of the mitral valve, explaining the negative isometric relaxation time found in these cases. This finding may help in the non-invasive assessment of the pulmonary vascular resistance of patients with transposition of the great arteries.