Disruption of visual percepts by a subsequent stimulus (ie, backward masking) has been consistently noted in schizophrenia, with some evidence that this fragility in early perception is present in people with genetic liability for the disorder. Given the potential of backward masking paradigms to mark neural processes that confer risk for schizophrenia, it is important to test the diagnostic specificity of abnormalities in visual perception. To more fully assess whether masking visual stimuli reveals a marker of genetic liability (ie, endophenotype) specific to schizophrenia, we tested 44 people with the disorder, 29 people with bipolar disorder, 56 first-degree biological relatives of people with schizophrenia, 26 first-degree biological relatives of people with bipolar disorder, and 43 nonpsychiatric control participants using a magnocellular-biased visual backward masking procedure that included target-to-mask onset asynchronies ranging from 0 to 80 ms. Relatives of people with schizophrenia who were without schizophrenia spectrum disorders exhibited impaired performance compared with nonpsychiatric control participants and relatives of people with bipolar disorder when a visual mask interrupted early perception (eg, 27 ms). A similar vulnerability of early processes was noted in people with schizophrenia, yet they also had impaired performance when masks occurred at later time points (ie, 80 ms). Performance deficits were not attributable to intellectual function, measures of attention and memory, symptomatology, or medication dosage. Bipolar patients and their relatives failed to exhibit deficits on the backward masking task. Fragility of early visual percepts appears to mark genetic liability specific to schizophrenia and may serve as an endophenotype for the disorder.