We surveyed 113 lakes in the northeastern U.S. in 2001 that had previously been sampled in 1986 to evaluate the effects of reductions in acidic deposition on the concentrations and speciation of aluminum (Al). We found ubiquitous decreases in the concentrations of total Al and inorganic monomeric aluminum (Ali) across the region. Median total Al decreased from 1.45 to 1.01 μmol L−1 across the region, with the largest decrease in the Adirondacks (4.60 μmol L−1 to 2.59 μmol L−1). Organic monomeric aluminum (Alo) also decreased region-wide and in all the subregions except the Adirondacks. The speciation of Ali shifted from largely Al−F complexes in 1986 to largely Al−OH complexes in 2001 in ponds whose concentrations were above the detection limit (>0.7 μmol L−1). In 2001, only seven lakes studied, representing a population of 130 lakes in the region, had Ali concentrations above a toxic limit of 2 μmol L−1 compared with 20 sample lakes, representing 449 lakes, in 1986. Thus, we estimate that more than 300 lakes in the northeastern United States no longer have summer Ali concentrations at levels considered harmful to aquatic biota.