The industrial area of Porto Marghera discharges every year about 1.85 10(9) m(3) of waste waters in the Lagoon of Venice through its 142 discharge points, 17 of them being constantly active. The Anti-Pollution Department of Magistrato alle Acque, the Venice Water Authority, has been controlling these discharges for many years. The huge database built up during the last years could help the authorities in making choices regarding the water quality of the Venetian environment. The application of chemometric methods to the dataset obtained from chemical analyses of industrial waste water samples (almost 250, for each of them up to 57 chemical variables having been measured) is useful to answer fundamental questions related to the pollution generated by the industrial area: i) which are the main differences among the individual discharge points? ii) is there a temporal trend in global and punctual pollution? iii) which is the discharge point having the strongest relative impact on the waters? The results of the present work allow to 1) identify two different groups of discharge points, discriminated by the level of contamination and by the presence of different contaminants; 2) detect a relevant temporal trend in one of the main outfalls (the industrial and civil waste treatment plant); 3) set up a multivariate strategy to "measure" the relative modification induced on receiving lagoon waters by a single discharge. The application of such a "3-STEP multivariate analysis" to the present and future data of water quality could represent a relevant tool for monitoring industrial activities, providing at the same time a support in management decision processes.