High-resolution (∼3 k.y.) δ18O records from middle Eocene mixed-layer dwelling planktonic foraminifera from the western North Atlantic show pronounced (>1‰) variability. The magnitude of change is greater than that seen in open-ocean Pleistocene records, but could not have been caused by ice-volume and/or sea-level fluctuations. Instead, the oxygen isotope shifts resulted primarily from large oscillations in sea-surface temperatures and indicate that the regional paleoceanography of the middle Eocene western North Atlantic was not consistently warm or stable. The large shifts in sea-surface temperatures could reflect variations in the position of the Gulf Stream relative to Blake Nose or variations in upwelling intensity.