Radon and radon daughter concentration was measured in natural caves, other underground cavities, and spas. Radon content of some cave waters, and 222Rn and 226Ra concentration of spa water samples were determined. Exposure and doses from inhalation of radon daughters were assessed for the staff and visitors. Systematic diurnal variation of radon concentration in some thermal spas, and seasonal variation of the radon level was found for most of the caves investigated. The diurnal variation of radon levels in spas was attributed to the operation of the bath. Caves with well isolated inner cavities showed more stable radon levels, which were only slightly affected by changes in the outdoor weather. The ventilation significantly reduced the radon concentration in spas. The effective dose values for the cave and spa personnel generally did not exceed the 20 mSv y −1 limit, recommended for workers by the ICRP 60 Publication. In cases when calculated dose was higher, it was recommended to the management that a more effective ventilation system or a reduction in occupancy be introduced in the high radon atmosphere. The doses for visitors and patients were one or two orders of magnitude lower than that of the personnel.