BACKGROUND Visual displays are often used to communicate important medical information to patients. However, even the simplest graphs are not understood by everyone. OBJECTIVE To develop and test a scale to measure health-related graph literacy and investigate the level of graph literacy in the United States and Germany. DESIGN Experimental and questionnaire studies. Setting. Computerized studies in the laboratory and on probabilistic national samples in the United States and Germany. Participants. Nationally representative samples of people 25 to 69 years of age in Germany (n = 495) and the United States (n = 492). Laboratory pretest on 60 younger and 60 older people. Measurements. Psychometric properties of the scale (i.e., reliability, validity, discriminability) and level of graph literacy in the two countries. RESULTS The new graph literacy scale predicted which patients can benefit from visual aids and had promising measurement properties. Participants in both countries completed approximately 9 of 13 items correctly (in Germany, x¯ = 9.4, s = 2.6; in the United States, x¯ = 9.3, s = 2.9). Approximately one third of the population in both countries had both low graph literacy and low numeracy skills. Limitations. The authors focused on basic graph literacy only. They used a computerized scale; comparability with paper-and-pencil versions should be checked. CONCLUSIONS The new graph literacy scale seems to be a suitable tool for assessing whether patients understand common graphical formats and shows that not everyone profits from standard visual displays. Research is needed on communication formats that can overcome the barriers of both low numeracy and graph literacy.