In Venice, where the average elevation is only a few decimetres, the term acqua alta ("high water') covers any temporary submergence of parts of the town by sea water. This phenomenon has become very frequent during the last two decades. Continuous records of significant tide values covering more than a century are now available. These make it possible to separate the effects of hydrological changes in the lagoon related to man's interventions from the influence of astronomical periodicities. The local rise in mean sea level since 1872 amounts to 27 cm, of which at least 14 cm are related to excessive pumping of water from underground sources. Another rise of about 12 cm can be ascribed to recent artificial morphological changes. In addition to this increasing trend, there is a 20 to 30 cm-wide oscillation, shown for the first time, over a period of 18.6 years related to the lunary declination cycle. The frequency and severity of flooding will increase until 1987. Then the situation will slowly improve again, until 1996.