Established in 2002, Agrobest is currently one of the largest agrocultural products manufacturers in Turkey, with 13% market share and 2 factories, located in Izmir and Manisa. Demand for agricultural products is both seasonal and highly uncertain. In their current production planning system, Agrobest focuses on maintaining maximum flexibility by keeping planning horizon short and making changes to production plans even within the frozen period. Due to the lack of long-term production planning and lack of strategical planning to minimize the effect of high demand uncertanity, Agrobest faces problem symptoms such as high quantity of lost sales and inefficient capacity utilization. The aim of our project is to design a decision support system for Agrobest’s long-term production planning that will minimize the cost of lost sales while taking demand uncertanity into account.