The primary aim of this study was to evaluate the effectiveness of standardized care package (CP) treatment for PTSD in a Danish sample of adult psychiatric outpatients (n = 948). Secondary aims were to identify baseline predictors of treatment outcome and to investigate differences in outcome between sex and between group and individual therapy. The naturalistic, non-randomized study followed a pre-post design. Patient data from five psychiatric outpatient clinics were collected from March 2011 and November 2017. Data were collected through questionnaires (SCL-90-R, WHO-5 and BHS) and therapist reported measures (GAF-S & GAF-F), at baseline and end of treatment. Effect sizes were calculated for all outcomes. Between-group effects for sex and therapy modality (group versus individual therapy) were analyzed with ANCOVA. Possible predictors of outcome were selected through LASSO regression and analyzed through hierarchical regression. Pre-post effects were small to moderate (d = 0.39 to 0.69). No differences were found in outcome between group and individual therapy, but women had significantly better outcomes than men. Besides sex, only baseline severity predicted outcomes. The effectiveness of the CP treatment was generally limited. This indicates a need for implementation of better therapeutic practices, such as evidence-based treatments and possibly better training of clinicians in the mental health sector. Our results underline the need for further research comparing group and individual treatment modalities using evidence-based therapies in RCT designs. It also emphasizes the need to investigate differences between sex to ascertain whether men and women have divergent treatment needs.