Decades of acidic deposition adversely affected aquatic ecosystems across eastern North America, with particular severe impacts noted in mountainous regions of New York State. The 1990 Clean Air Act Amendments (CAAA) reduced acid deposition and acidity of many poorly-buffered lakes in the Adirondacks, but had little effect on streams due to their dynamic hydro-geochemical regimes. Significant temporal trends and improvements in stream chemistry and biology did not become apparent until only recently. Water chemistry and discharge from six streams (1991-2017), and fish data from dozens of quantitative surveys (1979-2017) were assessed to ascertain the effects of the CAAA on acid-base chemistry and fish assemblages in acidified Adirondack and Catskill Mountain streams. Concentrations of sulfate and inorganic Al decreased, whereas pH and acid neutralizing capacity increased significantly in many previously acidified streams between 1991 and 2017. Inorganic Al concentrations decreased to, or below, an acute toxicity threshold of 2.0 µmol/L in several severely acidified streams and reached a chronic toxicity threshold of 1.0 µmol/L in some moderately acidified streams. Except for streams with barriers, total density and biomass of fish communities (and brook trout populations), and total community richness increased in de-acidifying streams between the early 1990s and present. These findings indicate that the chemistry and biology of many streams in acid-sensitive regions of New York are beginning to recover in response to the 1990 CAAA.