We developed an instrument to assess comprehension of informed consent information among 275 adults entering one of four ambulatory trials. At the conclusion of trial enrollment, subjects rated their understanding of the information presented and completed the Deaconess Informed Consent Comprehension Test (DICCT). Subjects completed the vocabulary subtest of the revised Weschler Adult Intelligence Scale (WAIS-R) and the reading subtest of the revised Wide Range Achievement Test (WRAT-R). The DICCT for 50 subjects was scored by 2 blinded investigators. Interrater agreement was 0.84 (df = 49, p < 0.001). To investigate the DICCT's potential validity, its scores were correlated with WAIS-R vocabulary scores (r = 0.44, df = 199, p < 0.01) and WRAT-R reading scores (r = 0.39, df = 268, p < 0.01). Understanding of consent information was rated as thorough by 70% of subjects. The mean +/- SD DICCT score was 20.4 +/- 3.9. The DICCT is a reliable instrument to assess comprehension of informed consent information. There is preliminary evidence for the scale's validity. The subjects believed that they had greater understanding of study information than was shown by the DICCT.