Studies evaluating the implementation of cognitive skills programs with mentally disordered offenders (MDOs) have reported high dropout rates, but little is known about the factors associated with treatment noncompletion. The authors analyzed data from the first randomized controlled trial of the Reasoning and Rehabilitation (R&R) program with MDOs to determine predictors of treatment dropout. Patients with psychotic disorders were recruited from medium-secure hospitals in the United Kingdom. Fifty percent of the 44 participants allocated to receive R&R prematurely terminated treatment. Program dropout was significantly predicted by scores on the Clinical and Risk Management scales of the Historical, Clinical, Risk Management—20, Factor 2 scores of the Psychopathy Checklist—Screening Version, antisocial personality disorder (ASPD), psychopathy, and recent violence. Psychopathy, ASPD, and recent violence emerged as the strongest predictors of dropout in multivariate analyses. Strategies to improve treatment retention should be targeted at MDOs who display high levels of impulsivity, poor behavioral controls, and antisocial or psychopathic traits.