It is well-known that directing attention to a location in space enhances the processing efficiency of stimuli presented at that location. Previous studies have shown that directing spatial attention manifests itself as an increase in spontaneous firing rate of neurons (the baseline signal) in extrastriate cortex at the retinotopic corresponding location. There has been considerable debate as to whether this preparatory effect of attention also occurs in human striate cortex (area V1). In the present study, participants had to direct attention to a cued location in space, while changes in blood oxygen level dependent (BOLD) signals were measured. We show that in conditions in which there was no change in sensory stimulation, modulations due to flexibly shifting spatial attention were present throughout early visual cortex (areas V1, V2 and V3). In all early visual areas, the increased BOLD signal in response to the cue was retinotopically specific. The present study shows that voluntary top-down attentional control modulates activity not only in extrastriate but also in striate cortex. This modulation occurs quickly and flexibly in a retinotopic fashion, and serves to facilitate target processing in a continuously changing environment.