<p>Radon is a natural source of radiation and one of the main causes of lung cancer. Since radon is a product of radium decay, it leaks from soil and rock and often accumulates in enclosed spaces. Caves are known for high radon concentrations, especially under certain meteorological conditions when the air circulation in the cave is weak. In this study, the results of radon monitoring in two caves in Croatia, Šparožna Cave and Biserujka Cave, are presented. Radon concentrations were measured continuously in each cave for 14 months. The monthly average concentrations showed a clear seasonal pattern in both caves, with much higher concentrations during summer. In some parts of the monitoring period in Šparožna Cave, diurnal changes in concentrations can also be seen. The highest measured concentration in the Biserujka Cave was 16 kBq/m³, while in the Šparožna Cave a maximum value of 47 kBq/m³ was measured, which is the highest radon concentration measured in any speleological object in Croatia. The effective dose for a 30&#xa0;min tourist visit to Biserujka was estimated between 0.015 and 0.053 mSv, depending on the month of the visit, while the effective dose for a 3-hour research visit to Šparožna Cave was estimated between 0.029 and 1.28 mSv. These results may help to provide recommendations for limiting the length of stay of employees and visitors in the investigated caves.</p>