It is suggested to spend some time in a cave for curing respiratory diseases. Some ascribe the healing effect partly to radon. In Hungary in the Hospital Cave of Tapolca the mean radon concentration shows 17 times difference in the winter and in the summer period. The change of the forced exhaled volume in 1 s (FEV1) values was examined measured by 1824 patients in this cave. By 70% of the patients the FEV1 value improved, by 30% it became worse but these were independent from the radon concentration of the cave. Therefore, radon concentration has no positive effect during the treatments performed in Tapolca.