Furnas Caldera Volcanic Complex, São Miguel Island (Azores), last erupted in 1630 and is famous for its intense hydrothermal activity (i.e. fumarolic fields, thermal springs, cold CO2-rich mineral waters and diffuse CO2 soil emanations), which directly affect the villages of Furnas and Ribeira Quente. Here we report the first systematic investigations and mapping of soil radon (222Rn) emanations in the Furnas Volcanic Complex, and we examine their potential health risks for local inhabitants. 222Rn in volcanic soils (60 cm depth) was repeatedly measured between 2005 and 2010 using a portable solid-state alpha detector (RAD7, Durridge Company Inc.). Results reveal a local background of 8000 Bq m−3 and several areas with anomalously high 222Rn activity (up to c. 400 000 Bq m−3), which coincide with advective or convective gas transport through volcanic structures and active fault zones. High 222Rn radioactivity in Furnas and Ribeira Quente villages represents a risk to the population. Continuous monitoring performed during November and December 2005 in a house in Furnas shows indoor 222Rn reaching 13 273 Bq m−3, two orders of magnitude greater than the reference level (150 Bq m−3), when the ventilation efficiency is reduced.