The paper analyses one of the longest contemporary wave measurements in the northern Baltic Sea, performed at Almagrundet 1978–2003. This record contains the roughest instrumentally measured wave conditions (significant wave height = c. 7.8 m) in the northern Baltic Proper until December 2004. The data for the years 1979–95, the period for which the data are the most reliable, show a linear rising trend of 1.8% per annum in the average wave height. The seasonal variation in wave activity follows the variation in wind speed. The monthly mean significant wave height varies from 0.5 m in May–July to 1.3–1.4 m in December– January. No corrections have been made in the analysis to compensate for missing values, for their uneven distribution, or for ice cover.