The water chemistry of aquifers and streams in the Upper Silesia Ore District, Poland are affected by their proximity to zinc, lead, and silver ores and by ongoing mining activities that date back to the 11th century. This report presents hydrologic and water-quality data collected as part of a collaborative research effort of the U.S. Geological Survey and the University of Mining and Metallurgy in Cracow, Poland to study Mississippi-Valley-Type lead-zinc deposits. MVT deposits in the Upper Silesia Ore District (Fig. 1) were selected for detailed study because the Polish mining industry allowed access to collect samples from underground mines and mine-land property. Water-quality samples were collected from streams, springs, wells, underground mine seeps and drains; and mine-tailings ponds. Data include field measurements of specific conductance, pH, water temperature, and dissolved oxygen and laboratory analyses of major and minor inorganic constituents and selected trace-element constituents.