In August 2009 a continuous cave-air temperature monitoring programme was initiated to help improve the understanding of natural microclimate conditions in Predjama Cave one of 10,700 karst caves in Slovenia. Whereas Predjama Castle, which is build partly in the shelter of one of the cave's entrances, is among the most popular and spectacular tourist destinations in Slovenia, Predjama Cave currently attracts fewer visitors. Studies of the cave microclimate are being conducted to help determine the potential impact of increased tourist visits on the natural cave environment. Recently recorded visitor numbers create no notable impact on the natural cave microclimate. However, increased tourist traffic and additional heat generated by proposed electrical installations could elevate air temperature in the cave. Provision of sustainable precautions to minimize such anthropogenic effects on the cave microclimate and cave fauna must be considered in any future planning for the development of Predjama Cave as a show cave. •Predjama Cave in Slovenia has big potentials for increased tourism in the future.•Continuous cave-air temperature monitoring initiated to understand microclimate.•Sustainable environmental precautions in future planning must be considered.