In this report, a patient is described with an occluded left circumflex artery, in whom the corresponding myocardium was protected at rest by sufficient collateral circulation. Because of angina pectoris class III, a PTCA of that occluded vessel was performed, complicated by a large dissection. Recruitable collateral flow, assessed from pressure calculations by a new technique, suddenly decreased at the very moment of dissection. This was accompanied by resting pain and ischemia on the ECG. This case report confirms the hypothesis that the collateral circulation can be damaged by PTCA and emphasizes that every PTCA implies a definite risk, even in case of an occluded coronary artery filled by collaterals.