Objective: The objective of this study was to investigate the importance of trust in researchers and other reasons that participating parents, former participants, and non-participants had for participating, or not participating, in a longitudinal cohort study on prediction and development of diabetes in children. Study design: A questionnaire addressing each of these groups, where respondents graded the importance of a set of listed reasons for participating/not participating, was randomly distributed to 2500 families in the All Babies in Southeast Sweden (ABIS) study region with children born between 1997 and 1999. Results: Lack of trust was not a central factor to a great majority of respondents who decided not to participate in the ABIS study or who later decided to opt out. Practical matters, like blood sampling and lack of time, were important factors to many more. Yet, four fifths of those who still participate in the ABIS study stated trust in the researchers to be an important factor to their initial decision to participate. Conclusions: Trust in researchers may be a necessary prerequisite in order for people to be willing to participate in research, but practical matters such as time that has to be spent or pain involved in collecting blood were more important factors than lack of trust in explaining opt out in relation to the ABIS study.