Coo vocalizations have been described in the communication systems of several macaque species. Their use has been suggested to reflect the emitter's "internal state", with regard to motivation for social contact. The idea that the production of Coos reflects a desire for contact has never been critically evaluated; the effectiveness of these signals in facilitating contact between the emitter and others has not been demonstrated. We designed a study to test the function of Coo vocalizations in adult stumptailed macaques (Macaca arctoides), using control measures similar to those first employed in research on reconciliation behavior in macaques (DE WAAL & YOSHIHARA, 1983). We found that the probability of an approach leading to friendly contact was significantly greater following a female's Coo vocalizations than it was during control trials. Analysis of the social context of the focal animal as it emits a Coo (its "initial state"), provides indirect evidence that Coos are "contact calls". We found that stumptails were significantly more likely to be alone than in contact with others when they emitted a Coo. Females were more likely to repeat their Coo vocalizations if they did not achieve friendly contact within 30 s. This result is in line with the view of Coo vocalizations as part of a feed-back loop with affiliative social contact as the goal state. Coos were produced by other adult females at a higher rate and with a shorter latency following the focal female's Coos than they were in control trials. Response vocalizations may affirm, to the "cooing" female, the presence and location of other troop members. On the basis of the non-vocal behavioral changes that these signals predict, Coos may be viewed as social tools, used to influence conspecifics, in addition to whatever their use may reflect regarding an emitter's "internal state". We suggest that the use of Coos to solicit approach may be a convention for the avoidance of aggression.