Clay mineral analyses of 300 surface sediment samples in the Skagerrak, Kattegatt, and the southwestern Baltic Sea have yielded qualitative evidence of considerable advective transport of suspended fine-grained terrigenous matter from the North Sea to the Baltic Sea. The transport is related to periodic near-bottom saltwater inflows, whereas the outflow of less saline surface water carries little suspended matter of Baltic origin. Although there are not enough data at this stage for exact budgeting, it is evident that this process contributes considerably to the sedimentation in the southwestern Baltic Sea and has probably done so during the past 6 k.y. It has been neglected so far. Hence, the southwestern Baltic Sea is believed to be a final sink for fine-grained North Sea material. This fact is corroborated by a compilation of sediment thickness data in the major basins of the Baltic Sea, which shows that the shallow southwestern basins accumulate more Holocene mud than the eastern and central basins. The external terrigenous matter also has far-reaching implications concerning the assessment of organic and inorganic pollutants, which are bound to fine-grained, clayey material. The Baltic Sea can no longer be regarded as a closed sedimentary system, and pollution assessment and control must be considered with a wider perspective.