Micro-bursts from TCP flows are investigated. The chiprate is introduced and used to quantify the short-term bitrate of TCP flows. This paper examines packets with chiprates above the 90th percentile. The examination is performed over time scales ranging from 244 ?s to 125 ms. Two issues are addressed, the impact and the causes of the micro-bursts. It is found that the packets with high chiprate experience an elevated probability of burst losses. For example, the probability of a burst loss is up to 10 times larger for packets sent in micro-bursts. Furthermore, in some settings, these packets experience higher loss rate in general. It is also found that micro-bursts cause an increase in queuing delay. The causes of these micro-bursts are investigated. One finding is that at short-time scales, ACK clocking, which should reduce micro-bursts, is not functioning correctly. For example, in some cases, most of the packets contained in micro-bursts are ACKed at a rate that is less than half of the data rate.