BACKGROUND: Bipolar Disorder (BD) and schizophrenia are included in the group of severe mental illness and are main causes of disability and morbidity in the local population due to the bio-psycho-social implications in patients. In the last 20 years or so, adjunctive psychological interventions been studied with the purpose of decreasing recurrences, stabilising the course of the disease, and improving the functionality in these patients. OBJECTIVE: To analyse the psychological effect of a multimodal intervention (MI) vs a traditional intervention (TI) program in BD I and schizophrenic patients. METHODS: A prospective, longitudinal, therapeutic-comparative study was conducted with 302 patients (104 schizophrenic and 198 bipolar patients) who were randomly assigned to the MI or TI groups of a multimodal intervention program PRISMA. The MI group received care from psychiatry, general medicine, neuropsychology, family therapy, and occupational therapy. The TI group received care from psychiatry and general medicine. The Hamilton and Young scales, and the Scales for the Assessment of Negative Symptoms (SANS) and Postive Symptoms (SAPS) were used on bipolar and schizophrenic patients, respectively. The scales AQ-12, TEMPS-A, FAST, Zuckerman sensation seeking scale, BIS-11, SAI-E and EEAG were applied to measure the psychological variables. The scales were performed before and after the interventions. The psychotherapy used in this study was cognitive behavioural therapy. RESULTS: There were statistically significant differences in socio-demographic and clinical variables in the schizophrenia and bipolar disorder group. There were no statistically significant differences in the psychological scales after conducting a multivariate analysis between the intervention groups and for both times (initial and final). CONCLUSION: This study did not show any changes in variables of psychological functioning variables between bipolar and schizophrenic groups, who were subjected to TI vs MI (who received cognitive behavioural therapy). Further studies are needed with other psychological interventions or other psychometric scales.