A combination of a circulation anda diffusion model has been developed to be used for dispersion studies in the Baltic Sea. As the time scale of interest is from months up to several years a straightforward way to model the dispersion would require the circulation model to be run for a very long time, which would be impractible. Instead atypical meteorological year has been constructed for which the circulation model has been run. The circulation model is three-dimensional, uses six layers in the vertical and hasa horizontal resolution of 10 kilometres. The diffusion model is of Monte Carlo type in all three dimensions. So far only passive pollutants have been treated but the model will be extended to include biochemical interaction and sedimentation processes. Results of applications to three outlets are shown.