[en] In Bulgaria underground uranium mining started during 1945, but radiation protection measures were not introduced until after 1956. Consequently a substantial number of miners were heavily overexposed. Up to now about 80 miners have died from lung cancer. First data based on over 50,000 measurements of radon and radon decay product concentrations regularly performed since 1956 are presented. At the beginning of the mining operations the exposure levels reached up to 180 WL. Even now at some workplaces the exposure exceeds the present limit of 0.3 WL up to 350%. The milling of uranium began in 1947, but the first tailing pond was built in only 1958. This delay caused severe environmental contamination. More than 10⁶m² of land had to be taken out of agricultural production. Summary results derived from tens of thousands of in situ field measurements and sample analysis are presented. At some locations the external exposure rate over the contaminated soil reached 10 μGy.h^-1 and the radium concentration in the soil up to 10,000 Bq.kg^-1. Owing to the high exposures the average probability of lung cancer death is significantly elevated for the miners and ranges from 0.15 to 0.24. (author)