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In the last decades, many studies have attempted to analyse the factors that may favour the evolution of cooperation. Among unrelated individuals, cooperation is expected to occur when partners exchange altruistic acts one another (i.e., reciprocity) or when the donor of an altruistic act may obtain secondary benefits from the act (e.g., increased reputation). The iterated prisoner's dilemma (IPD) is frequently used to analyse cooperation between two players. In this game, cooperation is constantly at risk of exploitation. Therefore, previous knowledge of the other player's attitude towards cooperation may positively affect an individual's decision to cooperate during the game. In various social species (including humans), group members may form friendly relationships that reduce uncertainty about the partner response and increase the mutual exchange of benefits. In light of this, we analysed whether humans cooperate more when playing an IPD with a friend than with a stranger. Each subject played an IPD twice, with a friend and a stranger, and had £10 per round to donate or not to the other player. Our results evidenced that humans cooperate more with, and donate more money to the other player when playing with a friend than with a stranger. Moreover, in game two subjects playing with unfamiliar players followed a raise the stakes strategy (RTS), i.e. they donated more money as the game progressed. The sense of trust in the other player's willingness to reciprocate the altruistic act is probably an important pre-requisite favouring cooperation and it may explain the differences between friends and unfamiliar players. The use of a RTS strategy in game two, however, indicates that subjects were attempting to increase the level of cooperation with unfamiliar players. These findings are consistent with the observation that friendship (and the capacity to build cooperative relationships) may be beneficial to individuals in many social animals and in humans.