Very few measurements have been done on wide area ATM networks. In this paper, we first look at how a compliant traffic can be generated with standard ATM equipment. We show that in our environment, even with a 23.42 Mbps VP out of 34 Mbps links, it is necessary to use some spacing in the ATM adapters to generate a compliant traffic. Our measurements show that with a large window TCP is able to achieve a throughput of 19 Mbps when the ATM level traffic is compliant with the traffic contract. However, if the ATM level traffic is not entirely compliant the segment loss rate increases quickly, and the TCP throughput collapses. A 3% change in the cell rate of the ATM level traffic can cause the TCP throughput to drop from 19 Mbps down to 1 Mbps.