One of the most effective treatments of asthmatic and other respiratory tract illnesses is the cave therapy. The hospital cave in Tapolca (Hungary) has been used for treatments of patients suffering from asthma since decades. In summer 222Rn concentration may even reach 20 kBq/m3 (17 times higher than winter), the role of radon in the treatment of asthma is still not clarified. During the cave therapy the inbreathing air with high radon-content causes the significant radiation dose to the patients. Daughter elements of 222Rn (210Pb; 210Po) with longer half-lives may accumulate in blood increasing the radiation dose of human body. The change of 210Po concentration in blood samples taken from the patients were examined in winter and summer months. 210Po concentration of blood samples was measured with semi-conductor detector α-spectrometer. The samples were solved with HNO3, HCl and sources were prepared by spontaneous deposition on stainless steel. To increase measurement accuracy 209Po tracer was added to the samples. In 63 samples the measured 210Po activity concentrations were between 0.035-0.800 mBq/g. Compare the concentrations of the blood samples taken before and after the treatment it was found that they increased in each cases. Based on this result it was possible to calculate the absorbed dose of the patient during the treatment.