It is well known that plasma urea (PU, mg/dl) is a good indicator of protein status in dairy cows fed conserved forages, and also that this relationship may differ between conserved forages and fresh herbage. The objective of this study was to relate plasma urea of grazing dairy cows to herbage, supplements, diet, grazing and cows characteristics. A dataset of 8 experiments carried out between 1993 and 2014, in which herbage intake was measured individually, was used. Herbage intake was measured from the faecal output (Cr2O3 or Yb2O3)/faecal index (N and ADF) method, or by the n-alkanes method. Diet crude protein (CP) concentration (dietCP, g/kg DM) was determined from measured intake and CP concentration of each feed, including grazed herbage. Supplements (conserved forages and/or concentrates) represented 12% of the diet (0 to 34%). Multiple regression analyses including cows, feeds, diet and grazing variables, either with individual cow × period data (n=709), or with data averaged per herd and period (n=114), showed that dietCP explained almost all the PU variability. Addition of cows (weight, milk production, daily DM intake), supplements (amount, nature, proportion of diet), or pasture and grazing (fibre concentration, digestibility, sward height, herbage allowance, daily access time to pasture) characteristics did not improve the model. Plasma urea and dietCP averaged 21 mg/dl (6 to 43) and 171 g/kg DM (106 to 246), respectively. At herd level, PU may thus be predicted from dietCP only: PU = 0.236 dietCP – 20, R2=0.82, s.d.=3.8. This equation means that any increase of 10 mg/dl of PU originates from an increase of 42 g CP/kg DM of the diet. Plasma urea may also be used as an indicator of dietCP, with dietCP = 3.47 PU + 99, R2=0.82, s.d.=14.6. This equation suggests a total urea recycling when dietCP is close to 100 g/kg DM, with no more urea in plasma. When measured simultaneously in the morning before feeding, PU and milk urea were strongly correlated, with a slope of one and an origin of zero. It is concluded that plasma urea or milk urea may be good indicators of diet crude protein concentration in grazing dairy cows, and vice versa.