When attempting to identify an object based on smell alone, people often visualize the perceived source of the odorant. This close association between olfactory and visual functions is supported by neuroimaging studies demonstrating activation of visual cortex during performance of purely olfactory tasks. Such activation might simply reflect the correlation between olfactory percepts and the corresponding visual images, or it might reflect a causal contribution of visual processing to olfactory perception. Here we provide evidence in support of the latter possibility. Using repetitive transcranial magnetic stimulation, we show that stimulating human visual cortex improves performance on a task requiring discrimination among different odor qualities. No significant improvement is found for tasks involving discrimination between intensities of the same odor, from stimulation of auditory cortex, or from "sham" stimulation. These results are thus consistent with a specific visual cortical influence on high-level olfactory perception. They also demonstrate that unimodal perceptual tasks are influenced by processing within cortical areas of other, seemingly unrelated, sensory systems.