An abandoned underground coal mine complex in southeastern Ohio was hydraulically sealed in 1980 by a 300-m long subsurface clay dike and mine-entry seals near the down-dip coal outcrop. Clay plugs also were emplaced in separate entries at a higher elevation than the dike. The intent was to flood the mine to decrease acid mine drainage. A few months after construction, an entry-seal clay plug blew out at an elevation corresponding to 85-90 percent inundation, and drains in natural materials supporting the dike began flowing, indicating leakage through or around the dike. Given these early setbacks, the objective of this study was to assess the sealing project after 20 years. Seasonally, inundation can reach 85-90 percent, but high water levels are transient because of leakage. Nevertheless, mine water chemistry indicates partial suppression of pyrite oxidation. Since sealing, mine water pH increased from 2.7 to 5.3, conductivity decreased from 2700 to 600 mu S/cm, and DO is <2 percent saturation. In a stream receiving the mine drainage, acidity and metals concentrations are highest at low flow due to release of stored mine water, but acid and metal loadings are lowest at those times. The improvement in water quality due to sealing is distinguished from annual variance or natural attenuation by using a nearby unsealed mine as a control. Factoring out pre-sealing differences between the two mines, decreases in acidity loading due to sealing are significant, and the sealing project has been beneficial.