Very high concentrations of naturally occurring radionuclides are encountered in Finnish groundwaters and wells. Radon ingested through drinking water can cause considerable radiation to the stomach. We assessed the effect of natural uranium and other radionuclides in drinking water on the risk of stomach cancer. Subjects (n = 144,627) in the base cohort had lived outside the municipal tap water system during 1967-1980. A subcohort of 4,590 subjects was formed for use as a reference group by random sampling of the base cohort, with stratification by age and sex. Within the subcohort, 371 subjects had used drinking water from drilled wells prior to 1981. Stomach cancer cases within the subcohort were identified through a cancer registry, and cases using water from drilled wells were selected. Activity concentrations of radon, radium-226 and natural uranium in the drinking water were analyzed using radiochemical and alpha spectrometric methods. The median activity concentration of radon in well water was 130 Bq/l for both the 88 stomach cancer cases and the 274 subjects in the subcohort. Median radium concentrations were 0.007 Bq/l for cases and 0.010 Bq/l for the subcohort, with a median uranium concentration of 0.07 Bq/l for both groups. Risk of stomach cancer was not associated with exposure to radon or other radionuclides. The hazard ratio of stomach cancer was 0.68 for radon (95% CI 0.29-1.59 at 100 Bq/l water), 0.69 per Bq/1 for radium-226 (95% CI 0.33-1.47) and 0.76 per Bq/1 for uranium (95% CI 0.48-1.21). Our results do not indicate an increased risk of stomach cancer from ingestion of radon or other natural radionuclides through drinking water at these exposure levels.