Randomized trials were performed in Denmark and The Netherlands to determine the effect of mailed reminders on the response rate in surveys among patients in general practice. In both countries, general practitioners handed out questionnaires to 200 adult patients who came to visit them. An intervention group of 100 patients received reminders at 3 weeks after the visit, whereas a control group of the remaining 100 patients did not receive reminders. The response rate was significantly higher in the intervention groups than in the control group in The Netherlands (86% versus 55%, respectively) but not in Denmark (87% versus 81%, respectively). Mailed reminders can improve the response rate in surveys related to a general practice, but they are not effective in all situations.