Development of a diagnostic test to estimate soil N supplying capability is a continuing research need. The Illinois soil nitrogen test (ISNT) has been proposed as a method for adjusting corn (Zea mays L.) N recommendations to account for soil organic N contributions. We evaluated the ISNT as a tool for predicting corn N response in Wisconsin by comparing ISNT values and corn N response data from 80 experiments conducted between 1984 and 2004 with a range of crop rotations, management histories, and soils. Relationships between various hydrolyzable soil N fractions (including amino sugar-N) and corn N response data were evaluated using a subset (13 sites) of the 80 N response experiments selected to obtain a wide range of anticipated soil N availability. Results showed that ISNT values were not related to observed economic optimum N rates (EONR) in the corn N response experiments and that the ISNT had no ability to separate N responsive from non-responsive sites. ISNT values were well correlated (r2 = 0.88) with the soil organic matter content of the experimental sites suggesting that the ISNT is measuring a constant fraction of the soil organic N rather than the readily mineralizable N component. Soil organic N fractions measured in 13 experiments were not related to corn N response although these experiments included cropping systems ranging from first year corn following alfalfa to continuous corn. Results from this work indicate that the ISNT and the soil organic N fractions studied are not reliable predictors of corn N response.