Six women suffering from Wolff-Parkinson-White syndrome and who were previously asymptomatic or nearly asymptomatic, were followed prospectively during and after their subsequent pregnancies. Three women experienced several attacks of supraventricular tachycardia (SVT) for the first time and the rest experienced an increase in their attacks. It seems that there is an increased tendency for SVT in pregnancy. Increased adrenergic sensitivity by estrogens, increased plasma volume, stress, and anxiety during pregnancy may be some of the causative factors.