TCP Slow Start exponentially increases the congestion window size to detect the proper congestion window for a network path. This often results in significant packet loss, while breaking off Slow Start using a limited slow start threshold may lead to an overly conservative congestion window size. This problem is especially severe in high speed networks. In this paper we present a new TCP startup algorithm, called Paced Start, that incorporates an available bandwidth probing technique into the TCP startup algorithm. Paced Start is based on the observation that when we view the TCP startup sequence as a sequence of packet trains, the difference between the data packet spacing and the acknowledgement spacing can yield valuable information about the available bandwidth. Slow Start ignores this information, while Paced Start uses it to quickly estimate the proper congestion window for the path. For most flows, Paced Start transitions into congestion avoidance mode faster than Slow Start, has a signi.cantly lower packet loss rate, and avoids the timeout that is often associated with Slow Start. This paper describes the Paced Start algorithm and uses simulation and real system experiments to characterize its properties.