Finished drinking water samples were collected from 384 waterworks that supply 70.9% of the Norwegian population. For 97 municipalities where a majority of the population has had a stable drinking water supply from at least 1965, analytical results for Si, Al, Fe, Mg, Ca, Na, Mn, Cu, Zn, Ba, Sr, K, F-, Cl-, Br-, NO3-, SO4(2-), pH, electrical conductivity, total organic carbon (TOC) and colour are correlated with municipal rates for morbidity of 16 groups of cancer (1975-84), and for mortality of 17 groups of other diseases (1974-83). Several associations are found, some of which may be real, while others are incidental due to the large number of correlations involved. The ecological design of this study implies that cause-and-effect interpretations should be made with great care.