Based on data from the US Geological Survey's COALQUAL database, the mean concentration of mercury in coal is approximately 0.2 mu g g(-1). Assuming the database reflects in-ground US coal resources, values for conterminous US coal areas range from 0.08 mu g g(-1) for coal in the San Juan and Uinta regions to 0.22 mu g g(-1) for the Gulf Coast lignites. Recalculating the COALQUAL data to an equal energy basis unadjusted for moisture differences, the Gulf Coast lignites have the highest values (36.4 lb of Hg/10(12) Btu) and the Hams Fork region coal has the lowest value (4.8 lb of Hg/10(12) Btu). Strong indirect geochemical evidence indicates that a substantial proportion of the mercury in coal is associated with pyrite occurrence. This association of mercury and pyrite Probably accounts for the removal of mercury with the pyrite by physical coal cleaning procedures. Data from the literature indicate that conventional coal cleaning removes approximately 37% of the mercury on an equal energy basis, with a range of 0% to 78%. When the average mercury reduction value is applied to in-ground mercury values from the COALQUAL database, the resulting 'cleaned' mercury values are very close to mercury in 'as-shipped' coal from the same coal bed in the same county. Applying the reduction factor for coal cleaning to eastern US bituminous coal, reduces the mercury input load compared to lower-rank non-cleaned western US coal. In the absence of analytical data on as-shipped coal, the mercury data in the COALQUAL database, adjusted for cleanability where appropriate, may be used as an estimator of mercury contents of as-shipped coal.