A data base of monthly inflow of fresh water from rivers and land to the Baltic Sea and its subbasins is created. The data base covers the period 1950 - 1990 and is based on observations from the national hydrological services of the surrounding countries.
The main features of the data base are presented including river flow of selected rivers and total inflow to the Baltic Sea and its subbasins. Long term, seasonal and short term variabilities are analysed and the effects of hydropower development are identified. An earlier data base by Mikulski (1982) is used for comparison and extension of the record to cover the period 1921 - 1990.
It is concluded that the variability of inflow is great and that the decade 1981 - 1990 is the wettest in 70 years. The increase in runoff is mainly due to increasing river flow during the cold seasons. The effects of hydropower development are noticeable in the records for the Bothnian Bay and the Bothnian Sea.