Most research on return-to-work (RTW) has focused on musculoskeletal disorders. To study RTW in employees sick-listed with common mental disorders (CMD), e.g., stress, depression, and anxiety, the National Research Centre for the Working Environment initiated a study on ''Common Mental Disorders, Return-to-work, and Long-term Sickness Absence'' (CORSA). The aim of the study is (1) to identify predictors of RTW from the environmental, the individual, and the health-related domain and (2) to explore the RTW process based on study participants' experiences. The purpose of this paper is to present the study design and the characteristics of the participants, including analyses on non-response and the prevalence of major depression.