A novel criterion for digital synchronous communication system efficiency estimation is defined. All system attributes such as bit error probability, coding efficiency, initial system synchronization time, efficiency of detection of timing anomalies and system sync recovery, reliability of the lock indication, and the amount of redundancy insertion in the data stream for synchronization and error-correcting coding purposes are included in a single parameter called "system efficiency." The expression for this parameter is derived under the assumption of signal ergodicity and steady-state system operation. In its final form, this parameter seems obvious almost by inspection. Even so, until now, it has not been used in the analysis of communication system efficiency. The analysis of this parameter for several different systems is presented as an illustration.