Lakewater acidification in New York's Adirondack Mountains was quantified based on paleolimnological reconstructions of pre-1850 chemistry for 49 lakes. Paleolimnology provides a more accurate assessment of regional acification than was previously possible using water chemistry and model calculations alone. Historical depletion of lakewater acid neutralizing capacity (ANC) and increase in labile monomeric Al have been restricted to lakes that are currently low in ANC. Most currently acidic lakes have acidified recently and were not acidic prior to 1850. The percentage of lakes affected and magnitude of the inferred acidification are substantially smaller than generally believed. The results agree, however, with short-term recovery rates previously documented and with spatial patterns in lakewater chemistry across a depositional gradient, and illustrate the importance of watershed and/or in-lake processes in neutralizing acidic atmospheric inputs.