Phase image analysis of blood pool scintigrams was performed in 6 patients with Wolff-Parkinson-White (WPW) syndrome to determine the relationship of phase changes and abnormalities of atrioventricular conduction. The site of preexcitation was located by invasive endocardial mapping. The phase advance generally correlated with the electrophysiologically determined location of the accessory pathway. Factors which increase or decrease the amount of preexcitation also increased or decreased the advance of mean phase angles. Phase imaging is a useful noninvasive means for tentative location of accessory pathways in WPW syndrome.