Human and monkey studies clearly show that the anterior intraparietal area (AIP) is crucial for hand-related visuomotor transformations. Human AIP activates also during observation of hand actions, involving it in the mirror system. It is not known, however, whether its activation can also reflect a difference in the complexity of the observed action. In the present study we used functional magnetic resonance imaging (fMRI) to explore the activation of human area AIP during the observation of complex object-manipulation tasks (e.g. inserting a key in a lock and turning it) as compared to simple tasks (whole hand grasping of an object) executed with the left and the right hand in a first person perspective. The results show that, in general, both complex and simple tasks produced an activation of the fronto-parietal mirror system and that the activity of AIP in each hemisphere was higher during observation of the contralateral hand (hand identity effect). A Region-Of-Interest (ROI) analysis of the parietal activations responding to hand identity showed that each AIP was more active during the observation of complex with respect to simple tasks. In the right AIP this effect was stronger during observation of the contralateral hand, in the left AIP was strong during observation of both hands. This complexity-related property was not observed in the other activated areas. These findings support the concept that the observation of motor acts retrieves the internal representation of those same acts in the observer's motor system (direct-matching hypothesis based on the mirror neuron mechanism).