In this study, the effect of farm, time, season and health was evaluated on the composition of mare's milk sold in Flanders. The content of the analyzed components (i.e. fat, fatty acids, protein, lactoferrin, lysozyme, lactose and urea) differed significantly (p < 0.0001) between farms, at a given moment in time. Within each farm, large month-to-month variations for most milk components (p <0.01 to 0.0001) were observed. The variation over time between different farms was smaller. These findings indicate that the composition of the mare's milk consumer portions varies substantially between the different farms and also over time within each farm. Season, nutrition, udder health and worm burden are believed to contribute significantly to this variation.