Magnetic resonance imaging (MRI) of retracted blood clots embedded in nonretracted clots was used to follow their lysis with urokinase in a plasma milieu in vitro. The two types of clots that were imaged in the same plane differed in signal intensity on T2-weighted spin echo MR images throughout the 20-hour observation period. It was thus possible to delineate the contours of both clot types and measure their relative sizes by digital image processing. Lysis of retracted clots proceeded significantly slower than lysis of nonretracted clots. Our in vitro results suggest that MRI might prove useful in detecting thrombus retraction in vivo and in predicting the outcome of thrombolytic therapy.