Milk urea nitrogen (MUN) can be used as a tool for the nutritional and environmental management of dairy cows as an estimate of the efficiency of protein metabolism. Using MUN to milk protein ratio (MUN/PROT) was suggested to improve the prediction. Thus, the aim of this study was to better understand the relationships between MUN and MUN/PROT with non-nutritional and nutritional variables in commercial dairy herds. Data recorded by Valacta (Dairy Production Center of Expertise Quebec-Atlantic) for the years 2009 to 2011 was used and originally comprised 3,481,705 test-day records (275,758 cows in 3,140 herds). The analysis was restricted to data from Holstein cows, between 1 and 305 DIM, reducing the number of admissible records for the analysis to 1,339,156 (189,913 cows in 2,522 herds). Average (±SD) milk production, fat content and protein content were 30.8 ± 8.5 kg/d, 3.89 ± 0.60% and 3.25 ± 0.33%, respectively. MUN was statistically different between milkings (10.1 and 10.7 mg/dL for AM and PM, respectively; P < 0.001). MUN/PROT was also statistically different between milkings (31.1 and 32.1 mg urea-N/kg milk CP for AM and PM, respectively; P < 0.001). Average MUN was statistically different between parities (10.2 and 10.5 mg/dL for first and second or more lactation, respectively; P < 0.001). MUN/PROT was also affected by parity (30.7 and 32.0 mg urea-N/kg milk CP for first and second or more lactation, respectively; P < 0.001). Pearson correlations between MUN or MUN/PROT and diet variables were low, the highest correlation for both parameters being r = 0.16 (P < 0.001) with crude protein content. Both parameters were negatively correlated with milk fat percentage (r = −0.19 and r = −0.29 for MUN and MUN/PROT, respectively; P < 0.001) and with the estimated breeding value for protein content (r = −0.10 and r = −0.22 for MUN and MUN/PROT, respectively; P < 0.001). MUN/PROT was also correlated with daily milk production (r = 0.20; P < 0.001). These results show that in commercial conditions MUN and MUN/PROT are affected by factors other than diet composition.