A record of oxygen isotopes in biogenic opal, 4200 to 1200 calibrated years before the present, from a high-altitude proglacial lake on Mount Kenya, East Africa, exhibits short-term fluctuations on a time scale of centuries as well as long-term variations. The short-term fluctuations are attributed to changes in the glacier meltwater input, and the long-term variations are related to changes in lake temperature. The record indicates that the climate was warm in Equatorial East Africa from 2300 to 1500 years before the present.