Leaching of atmospherically deposited nitrogen from forested watersheds can acidify lakes and streams. Using a modified version of the Model of Acidification of Groundwater in Catchments, we made computer simulations of such effects for 36 lake catchments in the Adirondack Mountains of New York State. Our simulations bound the potential changes in regional-scale surface water acidification that might occur there over the next 50 years across broad scenarios of both nitrogen and sulfur deposition as well as ranges of times to watershed nitrogen saturation. Model projections indicated that nitrogen deposition may play a more important role in future acidification than has been previously considered and may rival or exceed potential effects of sulfur deposition, depending on how rapidly watersheds might become saturated with atmospherically deposited nitrogen.