The capacity of visual working memory has been extensively characterized, but little work has investigated how occupying visual memory influences other aspects of cognition and perception. Here we show a novel effect: maintaining an item in visual working memory slows processing of similar visual stimuli during the maintenance period. Subjects judged the gender of computer rendered faces or the naturalness of body postures while maintaining different visual memory loads. We found that when stimuli of the same class (faces or bodies) were maintained in memory, perceptual judgments were slowed. Our results suggest there is interference between visual working memory and perception, caused by visual similarity between new perceptual input and items already encoded in memory.