We investigate whether electoral monitors, who are in charge of assuring the fairness of elections, interfere with their outcome. More precisely, does the monitors' presence bias the results in favor of their own preferences? To do so, we construct a novel dataset from the raw voting records of the 2011 national elections in Argentina. We exploit a natural experiment to show that electoral observers cause, on average, a 1.5% increase in the vote count for the observers' preferred party, which can reach up to 6% for some parties. This bias, which appears under various electoral rules, occurs mainly in municipalities with lower civic capital and weakens the accountability role of elections. (C) 2016 Elsevier B.V. All rights reserved.