More than 100 3He/4He isotope ratios were measured in CO2-dominated gas samples collected during the 1997--1998 seismic crisis that shook the Umbria-Marche, Central Apennines, Italy. Although the gas emissions were located in a typical crustal environment the measured values delineate variable mixing proportions between mantle-derived and crustal helium. During the monitoring period anomalous 3He/4He ratios were observed, together with a significant increase of fluid outflows. The observed 3He/4He anomalies appear to be driven by stress-induced crustal deformations affecting micro-fracturing and bulk permeability and, in turn, by a seismogenic-induced modification of the crustal permeability of rocks. Since crustal deformations can also occur aseismically, the geochemical anomalies must be considered indicators of processes occurring below the surface, which could eventually help to estimate the probability increase of an impending earthquake, instead of pure empirical precursors. ¿ 2001 American Geophysical Union