When focusing attention on some objects and ignoring others, people often fail to notice the presence of an additional, unexpected object (inattentional blindness). In general, people are more likely to notice when the unexpected object is similar to the attended items and dissimilar from the ignored ones. Perhaps surprisingly, current evidence suggests that this similarity effect results almost entirely from dissimilarity to the ignored items, and it remains unclear whether similarity to the attended items affects noticing. Other aspects of similarity have not been examined at all, including whether the similarity of the attended and ignored items to each other affects noticing of a distinct unexpected object. We used a sustained inattentional blindness task to examine all three aspects of similarity. Experiment 1 (n = 813) found no evidence that increasing the similarity of the attended and ignored items to each other affected noticing of an unexpected object. Experiment 2 (n = 610) provided some of the first compelling evidence that similarity to the attended items - in addition to the ignored items - affects noticing. Experiment 3 (n = 1,044) replicated that pattern and showed that noticing rates varied with the degree of similarity to the ignored shapes but not to the attended shapes, suggesting that suppression of ignored items functions differently from the enhancement of attended items.