Health care workers often assume that patients who have completed a certain grade in school can read at that level. This study examines the relationships between patient reading ability, the last grade completed, and the reading ability necessary to comprehend commonly used written materials. We tested 528 patients during regular visits to seven outpatient clinics serving a predominantly indigent population. In addition, we analyzed the readability of 280 brochures and consent forms used in these clinics. Most patients had reading abilities on a level far below their last grade completed, while almost all materials tested were written on a level far above average patient reading ability. We conclude that patient reading ability should be routinely tested and that written materials should be developed on a level commensurate with patient reading ability.