Spa water samples taken from hot and cold springs throughout Taiwan were analyzed for waterborne radon concentrations using electret ion chambers. The highest radon concentration was detected at Yangmingshan National Park, where it is closed to the action level of 11.0 kBq m-3. Next comes a sea-water hot spring at Green Isle on the east coast of Taiwan. The spa water used by the nearby inhabitants may increase the indoor radon concentration by a factor of two in extreme cases.