Four experiments examined the sensitivity of visual short-term memory to visual pattern similarity. Experiment 1 showed that immediate serial memory for novel visual patterns was sensitive to similarity. Using an item reocgnition task, Experiment 2 showed that subjects learned the descriptions of sets of similar and dissimilar patterns at the same rate. But repeated presentations of patterns in a serial memory task again showed a marked and persistent similarity effect (Experiment 3). The final experiment showed the visual similarity effect in serial memory for patterns that had been previously learned. The results show that (a) serial memory for patterns is sensitive to visual similarity, (b) the visual similarity effect is not due to perceptual confusions but originates in memory, (c) there is a clear dissociation between item and order errors, and (d) the visual similarity effect survives articulatory suppression. Visual serial order memory and verbal serial recall appear to share several of these properties, suggesting that the same constraints govern serial memory in each modality. Implications for the understanding of short-term memory are discussed.