For more than 100 years, forest cover in Europe has increased substantially due to afforestation and natural forest expansion. The latter, resulting typically from farmland abandonment and subsequent secondary forest succession, has played a major role in marginal mountain areas and possesses various highly important environmental and economic consequences. In the Polish Carpathians, farmland abandonment is a spatially dispersed, locally specific process because of small farm and land parcel sizes, and its extent in the entire region is not well known. This study aims therefore to map current farmland abandonment in the Polish Carpathians and to identify its spatial determinants. To map farmland abandonment we detected secondary forest succession on abandoned fields using Airborne Laser Scanning (ALS) point clouds and topographic data in 230 sample areas (tiles) distributed throughout the Carpathian communes. To reveal the spatial determinants of current farmland abandonment, we investigated variables describing the accessibility and environmental, land cover, and socioeconomic conditions of communes, using best subset regression modelling. The results showed that 13.9% of agricultural land was abandoned and underwent secondary forest succession in recent decades. Topography (mainly slope gradients) and employment outside of agriculture were the two most important spatial determinants of farmland abandonment. Consequently, a substantial forest expansion may be observed in the coming decades throughout the Polish Carpathians if no counteractions are taken.