A lack of a consistent and valid approach to screening within the jail often hinders identification and treatment. Furthermore, screening instruments developed for jail populations are often inadequate in detecting serious depression and anxiety disorders in women. While the remedy thus far has been the use of separate screening instruments for men and women, others have suggested that the K6, a six-item measure validated in large epidemiologic studies, may hold promise. Building on prior research, this study assesses the validity of the K6 in detecting depression, posttraumatic stress disorder, and anxiety disorders among 494 male and 515 female jail detainees. The authors found that 15% of males and 36% of females meet criteria for serious mental illness on the K6, with receiver operating characteristics–area under the curve scores of .84 and .93, respectively. This study not only establishes the validity and efficiency of using the K6 for screening within jails but also suggests a need for adjusting scale cut points.