In pure alexia (PA)—an acquired reading disorder consequent on posterior left-hemisphere stroke—the hallmark is a pronounced and abnormal impact of word length on reading speed. Some patients with semantic dementia (SD)—a neurodegenerative condition affecting semantic memory—have also been reported to show an abnormal word length effect (AWLE) in reading, even though they are not thought to have the basic visual-processing deficits hypothesized to underlie this phenomenon in PA. In the current study, an AWLE in reading was consistently observed in both PA and SD patients, but further manipulations demonstrated marked differences between the groups in the conditions that produce the length effect, its specific manifestation, and the pattern of other deficits accompanying it. All of the results are compatible with the twin hypotheses that the AWLE in reading arises from a visual-processing deficit in PA but from reduced top-down lexical/semantic support for word identification in SD. In other words, the AWLE in the two patient groups appears to be a common symptom arising from different underlying deficits: one bird with two stones.