In multiuser wireless networks, opportunistic scheduling can improve the system throughput and thus reduce the total completion time. We explore the possibility of reducing the completion time further by incorporating traffic information into opportunistic scheduling. In particular, we first establish general properties for opportunistic scheduling with file size information. Then, we develop new traffic aided opportunistic scheduling (TAOS) schemes by making use of file size information and channel variation in a unified manner. We also derive lower and upper bounds on the total completion time. Our results show that the proposed TAOS schemes can yield significant reduction in the total completion time.