Intensive longitudinal (IL) measurement, which involves prolonged self-monitoring, may have important clinical applications but is also burdening. This raises the question who takes part in and successfully completes IL measurements. This preregistered study investigated which demographic, personality, economic, social, psychological, or physical participant characteristics are associated with participation and compliance in an IL study conducted in young adults at enhanced risk for psychopathology. Dutch young adults enrolled in the clinical cohort of the TRacking Adolescents' Individual Lives Survey (TRAILS) were invited to a 6-month daily diary study. Participant characteristics came from five earlier TRAILS assessment waves collected from Age 11 onwards. To evaluate participation, we compared diary study participants (N = 134) to nonparticipants (N = 309) and a sex-matched subsample (N = 1926) of individuals from the general population cohort of TRAILS. To evaluate compliance, we analyzed which characteristics were related to the proportion of completed diary entries. We found that participants (23.6 ± 0.7 years old; 57% male) were largely similar to nonparticipants. In addition, compared to the general population, participants reported more negative scores on nearly all characteristics. Internalizing problems predicted higher compliance. Externalizing problems, antisocial behavior, and daily smoking predicted lower compliance. Thus, in at-risk young adults, who scored lower on nearly every positive characteristic and higher on every negative characteristic relative to the general population, participation in a diary study is unbiased. Small biases in compliance occur, of which researchers should be aware. IL measurement is thus suitable in at-risk populations, which is a requirement for its usefulness in clinical practice. (PsycInfo Database Record (c) 2023 APA, all rights reserved).