A nationwide study on the natural radioactivity of drinking water was made in Finland. The study was directed mainly at drinking water distributed by water supply plants with more than 200 users. Additional samples were collected from private dug and drilled wells. The samples were analyzed for 222Rn, 226Ra, gross .alpha.- and gross .beta.-activity. Half of the samples collected from drilled wells were also analyzed for U. The radioactivity of drinking water distributed by the water supply plants was on the average low, the mean concentration being 670 pCi/l for 222Rn and 0.1 pCi/l for 226Ra. The most radioactive water was found in drilled wells in which the mean concentrations were 17,000 pCi/l for 222Rn and 2.9 pCi/l for 226Ra. Some drilled wells also had abnormally high concentrations of U, up to 2100 .mu.g/l and even higher in some wells in the Helsinki region.