This paper addesses the problem of formation control in multi-agent systems (MAS) and adopts an event-triggered strategy to reduce the number of communications between agents. For that purpose, to evaluate its control input, each agent maintains estimators of the states of the other agents. Communication is triggered when the discrepancy between the actual state of an agent and its estimate reaches some threshold. The impact of additive state perturbations is studied. A condition for the convergence of the MAS to a stable formation is studied. Simulations show the effectiveness of the proposed approach.