Purpose. It has been suggested that crowding, the adverse low-level effect due to the proximity of adjacent stimuli, explains slow reading in low-vision patients with absolute macular scotomas. According to this hypothesis, crowding in the vertical dimension should be released by increasing the vertical spacing between lines of text. However, studies with different experimental paradigms and only a few observers have given discrepant results on this question. The purpose of this study was to investigate this issue with a large number of patients whose macular function was carefully assessed. Methods. MP1 microperimetry examination was performed for each low-vision patient. Only eyes with an absolute macular scotoma and no foveal sparing (61 patients with AMD, 90 eyes; four patients with Stargardt disease, eight eyes) were included. Maximal reading speed was assessed for each eye with French sentences designed on the MNREAD test principles. Results. The effect of interline spacing on maximal reading speed (MRS) was significant although small; average MRS increased by 7.1 words/min from standard to double interline spacing. The effect was weak irrespective of PRL distance from the fovea and scotoma area and regardless of whether an eccentric island of functional vision was present within the scotoma. Conclusions. Increasing interline spacing is advisable only for very slow readers (<20 words/min) who want to read a few words (spot reading). Vertical crowding does not seem to be a major determinant of maximal reading speed for patients with central scotomas.