An optimistic scheme, called ODL, that uses dummy locks to test the validity of a transaction for concurrency control in distributed database systems is suggested. The dummy locks are long-term locks; however, they do not conflict with any other lock. By the use of long-term dummy locks, the need for the information about the write sets of validated transactions is eliminated, and during the validation test only the related sites are checked. Also, the transactions to be aborted are immediately recognized before the validation test, and therefore the cost of restarts is reduced. The usual read and write locks are used as short-term locks during the validation test. This use of short-term locks eliminates the need for the systemwide critical section and results in a distributed and parallel validation test.