The Tam Giang-Cau Hai Lagoon, the largest in south east Asia, suffers from a great anthropogenic pressure and appears subject to a process of progressive environmental deterioration. To establish causes, history and trends of lagoon contamination, sediment samples representing three major parts of the systems were sampled and analysed for porosity, organic carbon, total nitrogen, delta(13)C, grain size, radiotracers and PCBs. PCB surficial concentrations range from 10.2 to 24.5 microg kg(-1), and maximum values are close to NOAA ERL guidelines. The downcore distribution of PCBs shows two well defined peaks in the core from Tam Giang (25.5 and 16.2 microg kg(-1)), whereas the sediment from Cau Hai is characterised by a maximum at the surface. Therefore, present trends indicate that contamination is still increasing or slightly decreasing. The prevailing congeners are 3-CB and 4-CB, hence recent PCBs appear to be mostly originated by Aroclor 1016- and 1242-like mixtures, with some samples influenced also by Aroclor 1248. Present sources seem to be similar everywhere and probably they are widespread all over the territory of the Thua Thien-Hue province.