Radon (Rn-222) is a noble gas formed in the uranium path (U-238) as a decay product of radium (Ra-226). It is estimated to cause between 3% to 14% of all lung cancers, depending on the national average radon level and smoking prevalence. Radon molecules emit alpha radiation, which is characterized by low permeability through tissues, but due to its remarkably high energy, it has a high potential for DNA damage. The aim of our research was to assess the radon concentration inside the houses of patients with advanced lung cancer and to analyze their socio-economics status. The measurements of radon concentration were performed in 102 patients with stage 3B or higher lung cancer in the region of Lublin, Poland. One month of radon exposure measurement was performed with alpha-track detectors. In addition, patients filled in a detailed survey about factors that might influence the concentration of radon inside their houses. The average concentration of radon during the exposure of the detector in the residential premises of the respondents was at the level of 69.0 Bq/m3 [37.0-117.0]. A few significant correlations were discovered, e.g., higher levels of radon in countryside houses or in houses equipped with air conditioning. As radon exposure is a modifiable risk factor for lung cancer, it is extremely important to find factors that may reduce its concentration in dwelling places. Since our research was performed in houses of people with lung cancer, taking corrective actions based on our findings could prevent new lung cancer incidence in patients' flatmates.