We used a randomized trial to compare two polio vaccine pamphlets written on a sixth grade level--the vaccine information statement prepared by the Centers for Disease Control (CDC) and an easy-to-read pamphlet we developed (LSU)--for reading ability, comprehension and preference among 610 parents with a broad range of demographic characteristics. Parents at all reading levels and incomes preferred LSU (76% vs. 21%, P < 0.001). Although readers of LSU achieved significantly higher comprehension (65% vs. 60%, P < 0.05) this difference may not be clinically significant. The information items presented with instructional graphics were the only items on which differences in comprehension levels achieved both clinical and statistical significance. Comprehension was lowest for the CDC mandated information on risks and the National Injury Compensation. Our findings demonstrate that simplifying written immunization material and making it more suitable will increase appeal, but such modification may not raise comprehension to an acceptable level without use of instructional graphics. Health education materials intended for general parent populations, which are written on a sixth grade reading level, may not adequately educate parents or prepare them for a discussion with their physicians.