Climate fluctuations during the past millennium are relatively well documented(1). On a longer timescale, there is growing evidence of millennial-scale variability of Holocene climate, at periodicities of similar to 2,500 and 950 years (possibly caused by changes in solar flux)(2,3) and similar to 1,500 years (maybe related to an internal oscillation of the climate system)(4-6). But the involvement of deep water masses in these Holocene climate changes has yet to be established. Here we use sediment grain-size data from the Iceland basin to reconstruct past changes in the speed of deep-water flow. The study site is under the influence of Iceland-Scotland Overflow Water (ISOW), the flow of which is an important component of the ＇thermohaline＇ circulation that modulates European climate. Flow changes coincide with some known climate events (the Little Ice Age and the Mediaeval Warm Period), and extend over the entire Holocene epoch with a quasiperiodicity of similar to 1,500 years. The grain-size data indicate a faster ISOW flow when the climate of northern Europe is warmer. However, a second mode of operation is observed in the early Holocene, when warm climate intervals are associated with slower ISOW flow. At that time the melting remnant of land-based, glacial-age ice may have provided a sufficient source of fresh water to the ocean to reduce ISOW flow south of Iceland.