Peer-assisted content distribution technologies have been attracting attention. By using not only server resources but also the resources of end hosts (i.e., peers), we can reduce the offered load on servers as well as utilization of the access bandwidth of the servers. However, offered traffi to the network may increase because the traffi exchanged between peers passes across the network. Specificall, if individual peers send traffi disregarding underlay network topology and traffi conditions, the peer-assisted content distribution method may cause excessive traffi offered to the network and poor application performance. We thus investigated the impact of traffi caused by peer-assisted content distribution on the underlay network. We found that although peer-assisted content distribution disregarding underlay network topology causes 80120% additional traffi compared with the optimal case, i.e., content distribution using cache servers allocated optimally in the network, using underlay network topology enables us to achieve almost the same efficien network resource utilization as the optimal case. We also found that the peer-assisted approach can adaptively cope with change in the traffi demand matrix because uploaders in the network are generated according to the demand matrix in a self-organizing manner. This is because peers that have downloaded the content become uploaders so many uploaders are generated in the area where a large number of content requests exist according to the traffi condition; therefore, the content delivery traffi can be localized.