Mixed farming systems have potential agronomic, environmental and socio-economic advantages over specialized farming systems. This paper attempts to quantify these advantages for the Dutch province Flevoland. A mixed farming system at regional level is characterized by intensive cooperation between two or more specialized farms, each producing crop or animal products. To test the hypothesis that such a mixed farming system might improve sustainability of agriculture in Flevoland, nutrient balances, labour requirements and labour income were quantified for a specialized arable farm, a specialized dairy farm and both combined into a mixed farming system, exchanging land, labour and machinery. Scope for reduced biocide use in the mixed farming system was assessed in a qualitative way. In the mixed farming system, labour income per ha was 25% higher. Seventy percent of this increase could be explained through higher yields per ha of the profitable crops ware potato (Solanum tuberosum) and sugar beet (Beta vulgaris). The remaining 30% resulted from lower costs, mainly through a better utilization of available labour. Differences between the combined nutrient balance of both specialized farms and that of the mixed farming system were small. Indications of reduced biocide use in the mixed farming system could not be found. It was concluded that in a mixed farming system, it is possible to realize a higher income without increasing environmental pollution. Key factor is the ratio between animal and arable production, determining the extent to which crop rotations can be widened and the relative amounts of slurry that can be applied to grassland.