One of the major concerns in optical networks is the bandwidth underutilization problem. In fact, as WDM technology keeps maturing, there is a bandwidth gap between the transmission speed of a wavelength channel (over a Gb/s) and the capacity requirement of customers' connections. In this regard, building cost-efficient optical networks requires an efficient traffic grooming solution at the high speed optical access nodes. In this paper, we propose and evaluate a new concept of traffic aggregation in wavelength-division multiplexing (WDM) optical networks. Our objective is to reduce the network cost while preserving the benefits of all-optical wavelength-routed networks. In order to assess the efficiency of our proposal, all underlying network costs are compared. These costs include that of the transceivers required at node level as well as the number of wavelengths. Our results show that the proposed aggregation technique can significantly improve the network throughput while reducing its cost.