The examined materials have been: dolomitic limestone <i>pietra di Botticino</i> (Brescia), algal dolomicrite <i>pietra di Angera</i> (Varese), argillaceous sandstone <i>pietra Simona</i> (Darfo, Brescia), granites of <i>Montorfano</i> and <i>Baveno</i> (Lago Maggiore, Novara), calcitic and dolomitic marbles from Candoglia and Crevola (Novara), respectively. They are typical building stones used in historical monuments of Lombardy, northern Italy, although employed in wider areas. Ultrasonic tests clearly show that the velocities of longitudinal waves are different for <i>c</i>, <i>b</i> and <i>a</i> fabric directions of each stone, with a degree of anisotropy, A=<i>c</i>/0.5(<i>b</i>+<i>a</i>), characterized as follows: 0.96 for the Botticino stone, 0.83 (Angera stone), 0.80 (Simona stone), 0.82 (Montorfano granite), 0.87 (Baveno granite), 0. 89 (Candoglia marble) and 0.85 (Crevola marble). The relationships between the ultrasonic data of the quarry stone samples, and those of free stones or stoneworks in historical monuments have shown that the development of decay processes causes a reduction of the longitudinal wave velocities: relative decreases over 50% have been recorded. Thus, ultrasonic investigations may be considered as a suitable tool to defining the decay stage of stones.