In uniformly managed fields, high yielding areas and areas with less plant-available soil nitrogen (Np) may have lower grain crude protein concentration (CP) due to smaller amounts of nitrogen available per kg grain yield. With site-specific fertilization, more nitrogen could be applied to such areas to achieve a better and more even CP within the field. To investigate the need for this and the possibility of estimating fertilizer demand from historical data, within-field variations in CP, correlations with yield and Np and their consistency in spatial distribution between years were investigated. Measurements were performed during 1998–2000 at 34 sites within a 15-ha field in southwestern Sweden. Winter wheat (Triticum aestivum) was grown in 1998 and 2000 and spring barley (Hordeum vulgare) in 1999. CP was dependent on both yield and Np, but was also affected by other factors. The spatial distributions of CP and grain yields within the field were not consistent between years, probably because fertilization relative to optimum varied differently between sites in different years. In 1998, CP and grain yield were negatively correlated and CP was larger at sites with more Np. CP had a positive correlation with both yield and Np in 1999. In 2000, CP was lower where yield was higher at most sites, but did not correlate significantly with either yield or Np. The positive correlation between yield and CP was connected with nitrogen fertilization below optimum, whereas the negative correlation was connected with nitrogen fertilization near or above optimum at most sites.