Milk urea (MU) is a common tool used for evaluation of diet composition and feeding disorders and is related as well to reproductive performances, longevity, health and product quality (e.g. cheese making ability). In addition, the apparent relationships of MU with N excretion suggest that decreased MU will decrease environmental pollution with N. The Belgian Blue (BB) breed, originated in central and upper Belgium in the 19th century, is composed of two strains: beef Belgian Blue and dual-purpose Belgian Blue (DPBB). The DPBB cattle has been only moderately selected for milk production but can also be popular in organic and direct marketing farms in Belgium. Currently, efforts are undertaken to better describe MU, and other relevant traits, in this breed. The aim of this study was to estimate genetic parameters for MU concentration and its relationship with milk yield and compositions in DPBB cows. The data used were 16,935 test-day records of milk yield and compositions collected from 2014 to 2020 on 3,220 primiparous DPBB cows in 49 herds. A random regression test-day model was used to estimate genetic parameters. (Co)variance components were estimated with the Bayesian Gibbs sampling method using a single chain of 100,000 iterates. The first 20,000 iterates of each chain were regarded as a burn-in period. Mean (SD) of MU was 22.9 (10.1) mg/dl. Heritability estimates for daily MU was 0.08 (SD<0.01) and ranged from 0.07 to 0.11. The mean (range) of genetic correlations between MU and milk, fat and protein yields, fat and protein percentages were, respectively, -0.10 (-0.26 to 0.01), -017, (-0.33 to -0.10), -0.16 (-0.31 to -0.07), -0.07 (-0.16 to 0.07), and -0.02 (-0.26 to 0.00). These results indicate that selection on MU is possible with no effect on milk yield or compositions. However, relationships between MU and other important traits such as longevity, metabolic diseases, and fertility are needed. The current study is also a first step to do comparative GWAS analysis, potentially highlighting genomic ...