A fourth wave was initiated in 2011 with a pilot study to test the questionnaire. Since the 1990 questionnaire, many twins have been asked to participate in selected sub-studies which have focused on specific study questions and traits, but no unselected questionnaire survey had been conducted after that. While many follow-up outcomes can be assessed by medical register linkage-studies in Finland, many health-related factors need to be assessed by interview or questionnaires given to the participants. To test the acceptability of what turned out to be an extensive questionnaire, we sent 254 pairs with both members alive and 299 twins from pairs where only one twin was available (due to death, migration or lack of address of the other twin) a questionnaire in February-March 2011. The response rate was 70% for full pairs, but only 55% for ‘single’ twins. With approval from the ethical committee, we revised the invitation cover letter to the ‘single’ twins to better motivate their participation in the 35 year follow-up.
