Active-light reflectance sensors have shown to be an effective method to assess site-specific N need for variable-rate fertilizer applications in wheat. Several research groups, including ours, are investigating this technology for corn N fertilization. This 2004 study was conducted in Missouri on seven producer corn fields to compare environmental measurements of N from sensor-based, variable-rate N application with single-rate, whole-field N application. Measurements included economic optimal N rate (EONR), crop N yield use efficiency (YUE), N fertilizer recovery efficiency, and post-harvest soil nitrate measurements. Averaged over all fields, the sensor-based method reduced N fertilizer rate by 37 kg/ha. As N rate increased and approached EONR, YUE decreased for each of the seven fields. Yet within a field, EONR and YUE were spatially variable. Compared to conventional single-rate application, sensor-based application did reduce post-harvest soil nitrate for some sub-field areas with little or no reduction in yield. Based on these preliminary results, the use of active-light reflectance sensors to control in-season corn N fertilization in the U.S. Midwest is promising.