To explore the role of reminders in recruiting and maintaining participation in an online panel. 50,045 individuals from five German federal states were invited by regular mail to participate in the online study "Health-Related Beliefs and Healthcare Experiences in Germany." Those who did not respond to the first attempt received a postal reminder. Comparisons of sociodemographic characteristics and responses were made between first-attempt respondents and those who enrolled after the second letter. After the initial letter, 2,216 (4.4%, 95%CI: 4.3%-4.6%) registered for the study; after a reminder 1,130 (2.5%, 2.3%-2.6% of those reminded) enrolled. Minor sociodemographic differences were observed between the groups and the content of the responses did not differ. Second-attempt respondents were less likely to participate in subsequent questionnaires: 67.3% of first-attempt vs. 43.3% of second-attempt respondents participated in their fourth survey. Recruitment costs were 79% higher for second-attempt respondents. While reminders increased the number of participants, lower cost-effectiveness and higher attrition of second-attempt respondents support the use of single invitation only for studies with a similar design to ours when the overall participation is low.