Provided are a method, system, and program for determining whether data has been modified. One node associated with at least one data unit and a last version number is received. A data structure having a plurality of nodes is processed, wherein there is one node associated with each data unit and at least one node that is capable of aggregating one or more nodes, and wherein version numbers are associated with the nodes. A determination is made from the data structure of one node that is associated with the data units associated with the received node. A determination is then made that at least one data unit associated with the received node has been modified if the received last version number is less than a version number of the determined node.