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A case history is presented describing the ecosystem changes that accompanied the nearly 90% reduction of SO2 and metal particulate emissions from Sudbury smelters during the past 25 years. The instances of severe ground-level fumigations that caused acute damage to vegetation in an area of approximately 1,000 km(2) have been nearly completely eliminated. Significant improvements in water quality have also occurred in many of the estimated 7,000 acid-damaged lakes. Several species of acid-sensitive phytoplankton, zooplankton and insects have invaded lakes where improvements have occurred. Epiphytic lichens have reinvaded the former ''lichen desert'' that once extended out 7 lan from the smelters. Sensitive species such as Evernia mesomorpha and Usnea hirta now exist throughout the area. The vascular plant communities have been relatively slow to recover in the most severely damaged terrestrial areas. Metal-tolerant grasses (e.g. Agrostis scabra, Deschampsia caespitosa) were the first species to invade the barrens. Acid- and metal-contamination of soil, severe microclimate conditions, and the damaging effects of insect pests appear to delay recovery of terrestrial ecosystems. Recovery rates of aquatic ecosystems are also affected by a suite of physical, chemical and biotic interactions and many lakes remain severely damaged.