BANSAI - The BAltic and North Sea marine environmental modelling Assessment Initiative Two numerical ecosystem models are used to analyse the ecosystem health of the Baltic Sea in the year 2005 in relation to the target and reference levels in the planned Baltic Sea Action Plan of HELCOM. The results of models run at Finnish Institute of Marine Research (FIMR) and Swedish Meteorological and Hydrological Institute (SMHI) show that in all Baltic Sea regions there are locations which can be considered as eutrophication problem areas based on HELCOM (2006) classification. The same conclusion can be drawn also according to a comparison with Alg@line results from sea areas as Arkona Basin, Bornholm Basin, Eastern Gotland Basin, Northern Gotland Basin and Gulf of Finland. The winter concentrations of dissolved inorganic phosphorus (DIP) were high in all Baltic Sea regions except Bothnian Bay (and Bothnian Sea according to the SMHI model). In the Arkona Basin and the Bornholm Basin the DIP concentrations were exceptionally high according to Alg@line measurements. According to the models, the algae blooming was remarkable approximately on 30 % of summer days in the Arkona Basin, eastern regions of Eastern Gotland Basin (including Lithuanian open waters and Gdansk Deep), Gulf of Finland and northern Bothnian Bay. The chlorophyll a concentration of 2 mg m-3 (exceeding the HELCOM acceptable deviation levels) the concentration of 3 mg m-(indicating remarkable algal bloom) was exceeded for more than 60% and 30% of summer days, respectively, in the Arkona Basin, eastern regions of Eastern Gotland Basin (including Lithuanian open waters and Gdansk Deep), Gulf of Finland and northern Bothnian Bay according to both models. Alg@line 2005 chlorophyll a concentrations were lower than the Alg@line-references in the Arkona and Bornholm Basin, but higher than on Alg@line-references in other regions. The oxygen concentration in the bottom layers was under the critical limit (of 2 mg/l) for living organisms most of the year in northern parts of the Eastern and Western Gotland Basins and in the Northern Gotland basin.