The objective of this study was to determine how non-nutritional factors such as herd, parity, days in milk, milk production, and milk components affect the concentration of milk urea. A retrospective, observational study comprising analyses of individual cow records from monthly dairy herd improvement milk tests including milk urea tests was conducted. Six commercial Holstein dairy herds were involved in the investigation from October 2000 to September 2003. The data were evaluated using the mixed linear model with repeated measures, and parameters were estimated by the REML method. Significant differences in milk urea concentrations were observed between herds (P lt; 0.0001), with a significant variability between test dates within herds. Significanteffects of parity (P = 0.0003), days in milk (P lt; 0.0001), and their interaction were also found.