A site index prediction equation for oak stands was developed from data of 39 permanent sample plots in southcentral Pennsylvania. The utility of the equation, SI50 = B0 + B1 (slope position) + B2 (slope) + B3 (depth of A1 + A2 + B4 (silt) + B5 (Ca) + B6 (Mg) + B7 (N), further tested on 16 additional plots, compared favorably with a standard method of site index determination. The prediction equation accounted for 79 percent of the variation in site index. Calcium, magnesium, and nitrogen concentrations in the A1 + A2 soil horizon were very important to the prediction of site quality on these soils. Forest Sci. 18:192-195.