Synchronization of data items between parties A and B. A maintains copies of data items and an A change count and a B change count for each data item. The A change count indicates the number of changes made by A, and the B change count indicates the number of changes made by B and known to A. After a synchronization trigger, A sends to B each data item that A has changed, plus the A change count and the B. change count for each sent data item. A receives from B each data item that B has changed, plus the A change count and the B change count for each received data item. A detects a conflict if the received A change count differs from the maintained A change count.