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The present study was conducted to determine the relationship between milk urea nitrogen (MUN) and milk yield, fat and protein contents, breed and season effects, parity and days in milk in dairy cows from Paraná State, Brazil. A total of 127,428 test-days from 16,013 dairy cows belonging to 96 herds enrolled in an official milk recording program were analyzed. Multivariate mixed model methodology was used to determine the relationship between MUN and the fixed effects and the covariable milk production. Milk urea nitrogen averaged 14.45±4.60 mg/dL. Positive and intermediate association between MUN and milk yield (r = 0.34) were found. Holstein cows showed lower MUN adjusted means than crossbred, Jersey, and Brown Swiss cows: 14.18 vs. 15.49, 16.12, and 17.62 mg/dL, respectively. First-lactation cows showed higher MUN values than second-lactation and older cows: 16.16 vs. 15.95, and 15.45 mg/dL, respectively. MUN test-days collected during the winter were higher than those collected in the other seasons. The effects of the lactation stage on MUN were significant, with the highest MUN values observed in the sixth month of lactation. High-producing dairy cows showed higher milk urea concentrations but several environmental factors may contribute to reduce this important parameter of diet utilization efficiency.