Empirical data ultimately form the basis for most environmental studies. This work uses extensive data sets from the Baltic Proper on total phosphorus, total nitrogen, chlorophyll, temperature, and salinity. We present trend analyses to see whether there have been any changes in these variables between 1990 and 2005 (and between 1974 and 2005 for chlorophyll). To put these results into a wider context, we have used data from more than 500 systems throughout the world. Selected results: The total phosphorus and total nitrogen values in the Baltic Proper are fairly stable between 1990 and 2005. There are no major changes in surface-water temperatures or salinities this period. Chlorophyll shows a slowly decreasing trend in the surface-water layer since 1974. Compared to the situation in many other marine areas, the eutrophication in the Baltic Proper is moderate. The total river inflow of total phosphorus to the Baltic Proper is 30 to 40 kt/y; about 14 kt/y of total phosphorus come from the Kattegat and about 160 kt/y from land uplift. The inflow of total nitrogen to the Baltic Proper from the Kattegat is about 120 kt/y, and about 480 kt total nitrogen per year come from land uplift. Denitrification is about 660 kt/y. The total turnover of total phosphorus in phytoplankton is about 488 kt/y. The median monthly total nitrogen/total phosphorus ratio has been higher than 7.2 (the Redfield ratio) all months since 1994, indicating that the primary production in the system is generally limited by phosphorus rather than nitrogen. If the total nitrogen/total phosphorus ratio is lower than 15, there are increasing risks of blooms of cyanobacteria, especially if the water temperature is above 15°C. If the total nitrogen input to the Baltic Proper is lowered, it will mean a lowering of the total nitrogen/total phosphorus ratio and greater risks for blooming of harmful algae.