Mental health surveys are used extensively in epidemiological research worldwide. The ethical questions that arise regarding their risk of causing psychological distress or other potential harm have not been studied in the general population. We have investigated how study participants serving as controls in a population-based study perceived an anonymous postal questionnaire focusing on mental health and wellbeing. Parents were contacted from the Swedish Census Bureau as part of a larger follow-up study on palliative care conducted in 2001. Eligible parents had a child of the same gender, year of birth and were from the same counties in Sweden as parents who had lost a child to cancer. Five percent reported being negatively affected. The principle negative effect on participants was that self-reflection reminded them of their difficulties. Of the 418 respondents, 52% reported that they were positively affected by study participation and 95% perceived the inquiry as valuable. These findings support the use of population-based controls in future research.