Objective: The present study aimed to contribute to the design of effective health education information. Based on cognitive-psychological theory, pictures were expected to improve understanding of two existing textual instructions for using asthma devices (inhaler chamber and peak flow meter). From an analysis of the affordances and constraints of both devices this effect was expected to be stronger with the inhaler chamber than with the peak flow meter. Methods: To test this, both instructions were systematically illustrated with seven line-drawings visualizing the actions. In two separate randomized controlled trials with in total 99 participants from the general public, the original text-only versions were compared to the text-picture versions of the same instruction. Dependent variables were participants' recall of the instructions and the quality of their performance with the instruction observed from video-recordings. Results: Conform expectations, the results showed significant positive effects of pictures on recall and performance in both instructions, especially with the inhaler chamber. Conclusion: Thus, pictures may contribute to a better comprehension and use of medical devices that are inherently less clear. Practice implications: Health educators may optimize instruction design by careful analysis of the device with instruction and observational testing with potential users.