Abstract Year-long continuous radon monitoring was carried out (using Sarad Radon Scout devices) in a dwelling with high radon levels in the karst region of Slovenia. Two living rooms were selected: one on the ground floor with normal housework activities; and the second, on the first floor, closed and unattended. Meteorological data were also recorded. The following seasonal geometric means of radon activity concentration (kBq m −3 ) have been found: 6.28 ×/: 3.05 for spring, 1.25 ×/: 3.78 for summer, 5.17 ×/: 2.03 kBq m −3 for autumn and 9.83 ×/: 1.48 for winter on the ground floor; and 1.43 ×/: 3.71 for spring, 0.168 ×/: 2.49 for summer, 1.08 ×/: 2.39 for autumn and 2.08 ×/: 2.14 for winter on the first floor. Results are supported by additional radon measurements in other rooms; and in water the results indicate a strong radon source associated with an underground karst shaft.