In closed soilless growing systems there is a risk of spreading root pathogens over the nursery. Existing disinfection methods, such as heat or ozone treatment are rather expensive. Therefore, application is hardly feasible on nurseries smaller than 1 ha. Slow sand filtration may be an alternative to eliminate root pathogens. Aim of the experiments was to investigate the requirements at which slow sand filters have to meet for practical application in horticulture. Six treatments, namely three sand types (coarse, 0.5–1.6 mm; middle 0.2–0.8 mm; fine 0.15–0.35 mm, with an effective size of 0.71 mm, 0.51 mm and 0.23 mm, respectively) and two filtration rates (0.1 and 0.3 m3.m-2.h-1) were compared in two replicas in separated closed systems growing tomatoes in rockwool. Effectiveness was tested by inoculating Phytophthora cinnamomi, Fusarium oxysporum f.sp. Lycopersici and tomato mosaic virus. In addition, physical (EC, temperature) and chemical (COD, BOD5, NH4 , NO3-, pH and oxygen concentration) parameters were measured. P. cinnamomi was filtered out completely at the filters with the fine and middle grain size at a filtration rate of 0.1 m3.m-2.h-1 while it passed through all other filters. Fusarium oxysporum f.sp.Lycopersici and tomato mosaic virus could be detected in the effluent during a long period. Both pathogens were only retarded. Neither physical nor chemical parameters were influenced by the flow rate or the grain size. Only small differences between influent and effluent could be measured for NH4 and oxygen concentration.