The paper presents an innovative method of measurement of long-term deformations of bridges. The use of high precision electronic inclinometers permits an efficient and cost-effective automatic monitoring of bridge deformations. The system is easily applicable to structures subjected to intense traffic loads and is able to follow accurately the movements of the bridge during daily and seasonal temperature variations. The system has been in operation in the Riddes viaduct in Western Switzerland for over a year, and has provided valuable information on the behavior of the bridge under service condition. The aim of the measurements is to provide the owner with accurate information on the on the possible downward tendency of the main span. Because of extreme temperature movements, which occasionally exceed 50 mm within a single day, classical methods of measurements are inapplicable, or would at least require a very long period of observation. The system has been able to quantify that tendency within less than tree years of measurement. Preliminary results indicate that previous interventions on the bridge have mainly stabilized the downward deflection behavior.