Plant-based measurements can be used to diagnose the in-season N status of corn (Zea mays L.). Our objective was to assess the relationship between the corn nitrogen nutrition index (NNI), an index based on whole plant N concentration, and leaf nitrogen concentration of the uppermost collared leaf expressed on the basis of dry matter (DM; N-L) or area (N-LA). Three to seven N fertilization treatments were used in this study conducted at eight site-years in Canada and two site-years in France. Leaf N concentrations were measured from the uppermost collared leaf on three to five sampling dates along with NNI determinations. At all sites and most sampling dates, N-L, N-LA, and NNI increased with increasing N rates. With data from all sampling dates, N-L and N-LA were related to NNI, but the intercept of the response curves varied with site-year. These site-specific relationships should be used with caution for the in-season evaluation of corn N status. At stage of development similar to V12, however, N-L was strongly related to NNI (NNI = -0.035 + 0.028 N-L; R-2 = 0.82) and the response curve was not affected by site, year, nor cultural practice; N-LA was also related to NNI, but the intercept of the response curve varied with site-year. Determining N concentration (DM basis) of the uppermost collared leaf at, or near, the V12 stage of development is an alternate method for assessing NNI, hence eliminating the need to measure shoot biomass.