Background. Undetected Common Mental Disorders (CMDs) amongst people on sick leave complicate rehabilitation and return to work because appropriate treatments are not initiated. Aims. The aim of this study is to estimate (1) the frequencies of CMD, (2) the predictors of undetected CMD, and (3) the rate of return to work among sick listed individuals without a psychiatric disorder, who are registered on long-term sickness absence (LSA). Methods. A total of 2,414 incident individuals on LSA with a response rate of 46.4%, were identified for a two-phase study. The subsample of this study involved individuals registered on LSA who were sick-listed without a psychiatric sick leave diagnosis. In this respect, Phase 1 included 831 individuals, who were screened for mental disorders. In Phase 2, following the screening of Phase 1, 227 individuals were thoroughly examined by a psychiatrist applying Present State Examination. The analyses of the study were carried out based on the 227 individuals from Phase 2 and, subsequently, weighted to be representative of the 831 individuals in Phase 1. Results. The frequencies of undetected mental disorders among all sick-listed individuals were for any psychiatric diagnosis 21%, depression 14%, anxiety 4%, and somatoform disorder 6%. Conclusions. Undetected CMD may delay the initiation of appropriate treatment and complicate the rehabilitation and return to work.