Organization and resolution were studied histologically in 48 valve pocket thrombi from femoral veins. The great majority were essentially single structures and the overlying cusps were normal and uninvolved, Thrombus adherence to the distal part of the vein wall in the pocket is followed by cellular and vascular invasion and progressive organization. At the same time many thrombi continue to grow by proximal addition, whilst others regress fully to fibrous intimal plaques. Central fibrinolytic changes are frequent. Focal organic fragmentation following endothelial cell surfacing is not uncommon and is a potential cause of thrombus detachment.

Eight thrombi differed in that 5 were anchored to part of the cusp and 4 had a double structure resulting from two demarcated thrombotic events. One thrombus showed both features. In 3 thrombi with anchored cusps scarred intimal remnants of old thrombosis were present nearby. In 3 double structures recent thrombus had been laid down on organizing or organized thrombus, whilst the fourth was unique in that one part was composed solely of a large mass of platelets. The role of endothelium‐induced fibrinolysis in preventing cusp adherence and of intact endothelium in preventing second thrombi are discussed.