At the end of the 1980’s, a series of existing bridges in Switzerland were equipped with a network of hydrostatic leveling devices, with the aim of following their longterm behavior. Some of the bridges were known to exhibit a steady increase in their deformations, while others were considered in stable condition. The paper describes the measurement system, and the choices that were initially made to ensure that it would be durable. After more than twenty years of operation, the long-term experience related to the operation of this system is presented. The mode of operation of the system is discussed. The results of the monitoring are shown for several bridges. In the mean time, several bridges have been strengthened by additional post-tensioning, one bridge was even strengthened twice. The effect of the additional prestressing and the subsequent changes on the long-term behavior are shown.