Air and groundwater temperatures were measured in a rimstone pool in Postojna Cave, to advise evaluation of the impact of natural and anthropogenic influences related to heat being initially transmitted into cave air by visitors. Such heat can accumulate both in the rock mass and in water. Results show that attention must be paid to temperature changes of rimstone pool water rather than those of cave air, especially during the winter. Thanks to good ventilation deep inside the cave, short-term air temperature increases related to higher visitor numbers have not influenced flowstone precipitation or the cave fauna. However, this situation might change if winter visitor numbers increased greatly. This study does not support a suggested increase in winter visits (currently the tourism low-season) and reduction of summer visits (currently the tourism high-season). •Postojna Cave is one of the World's most-visited show caves, attracting around 500,000 visitors per year.•The potential impact of mass tourism on the cave air and water temperature was studied.•The susceptibility of cave water temperature to anthropogenic influences is higher than the susceptibility of cave air.•Higher visitor numbers during winter represent a greater disturbance to the cave microclimate than in summer.