Currently, there are many diagnostic self-tests on body materials available to consumers. The aim of this study was to assess the effect of an online decision aid on diagnostic self-testing for cholesterol and diabetes on knowledge among consumers with an intention to take these tests. A randomized controlled trial was designed. A total of 1259 consumers with an intention to use a diagnostic cholesterol or diabetes self-test were selected from an existing Dutch Internet panel. The intervention group was invited to view an online decision aid offering general information on self-testing and test-specific information on cholesterol and diabetes self-testing, including indications for testing, how to perform the test and interpreting the result. The placebo condition consisted of a limited information sheet. Multiple regression analysis showed that the knowledge level in the diabetes arm was higher in the intervention group than in the control group (B = 0.657; 95% CI, 0.011-1.303), as was the number of participants with an informed choice (OR, 1.672; 95% CI, 1.134-2.465). No differences were found in the cholesterol arm. Consumers who are considering doing a self-test should have access to independent information on self-testing and be encouraged to read this information.