The oxygen and carbon isotope composition of sedimentary carbonates formed in Lake Hayq, a closed-basin lake in northern Ethiopia, suggests marked variation in the balance between precipitation and evaporation during the last 2000 years. Diatom stratigraphy shows that the lake remained deep enough to support planktonic taxa, and was never saline. The record suggests that rainfall was higher than that of the twentieth century during most of this time, especially during two short intervals centred on AD 700 and AD 1300. Climate was drier only around AD 800, and from AD 1750—1900. Similar, but slightly moister climate than today, with high interdecadal variability, prevailed from AD 800 to AD 1200, equivalent to the European `Mediaeval Warm Period'. A period of high effective precipitation followed, from AD 1200 to AD 1700, during the `Little Ice Age'. Pollen evidence suggests that increased forest cover from AD 1400 to AD 1750 was a response to higher rainfall; forest cover may have increased evaporative loss from the catchment, contributing to a subsequent trend of increasing lakewater δ18O values.The record shows similarities in the timing and direction of changes in proxy-climatic records from Lake Naivasha (Kenya) and from the Nile.