BackgroundIn this study we evaluated the effectiveness of Regression Therapy (RT) to treat a broad spectrum of psychopathological and or Medical Unexplained Symptoms complaints (MUS). Regression therapy aims to identify and address the root cause of an individual's current emotional, psychological, or physical issues by accessing forgotten or suppressed memories that are present in the subconscious domain Methods A total of 169 subjects filled out a BSI-53 questionnaire before initiation of treatment as well as after 2 and 4 months of therapy. Fifteen established RT therapists conducted therapy according to a standardized protocol. Questionnaires were submitted anonymously to a commercial company that analysed the results and reported to an independent assessor. ResultsRT proved to be highly effective. The BSI-53 total score decreased from a value of 0.91 to 0.48 after 4 months of treatment. A score of 0.48 is close to the value of 0.42 measured in a representative sample of the Dutch population. For most study participants a single session was already satisfactory. Hedges g for this study was 0.95 indicating a large clinical effect. All 9 domains of the BSI-53 questionnaire showed a similar decrease in score, the mean decrease was 47%. Application of unbiased machine learning to all study parameters revealed that a positive effect of the therapy could be predicted from a few BSI-53 questions with high accuracy (AUC: 0,87). Limitations This blinded outcome trial was not compared to an active control.ConclusionThis study shows that RT is a highly efficacious treatment modality. It is fast, a few sessions already strongly alleviate psychological and physical symptoms.