In wavelength-division multiplexing (WDM) optical networks, the bandwidth request of a traffic stream can be much lower than the capacity of a lightpath. Efficiently grooming low-speed connections onto high-capacity lightpaths will improve the network throughput and reduce the network cost. In this paper, we propose and evaluate a new concept of traffic aggregation in WDM mesh networks that aims to eliminate both the bandwidth under-utilization and scalability concerns that are typical in all-optical wavelength routed networks. This approach relies on the multipoint-to-point lightpath concept. In order to assess the efficiency of our proposal, all underlying network costs are compared. To achieve this aim, we devise a new provisioning algorithm to map the multipoint-to-point lightpaths in the network. Our results show that the proposed aggregation technique can significantly improve the network throughput while reducing its cost.