Lake carbonates are generally assumed to precipitate at or very close to oxygen isotopic equilibrium with the ambient water. We present results from the eutrophic Lake Arresø, northeastern Sealand, Denmark, that demonstrate disequilibrium precipitation of present-day calcite. According to the temperature equation of Epstein et al., these calcites are depleted in 18O by 2‰–3‰ when compared to the expected equilibrium values. Core studies indicate that the development of disequilibrium conditions in Lake Arresø is correlated with the onset of an enhanced eutrophication. A high precipitation rate is suggested as the mechanism responsible for the observed 18O depletion of the calcite precipitates.