The goal of this study is to investigate the therapeutic efficacy of Operational Brief Psychotherapy - OPB. The instruments employed were Psychological Interviews according to the Adaptive Operationalized Diagnosis System and Operationalized Brief Psychotherapy - OPB. An experimental group (n=30) was compared to a Control group (n=30) through the use of the Operationalized Adaptive Diagnostic Scale, OADS - at the beginning of the study and the end of a three months period for both groups, formed by male and female adults. The experimental group (randomly formed) was submitted to Operationalized Brief Psychology and the Control Group (constituted by patients in the line for the College's Psychotherapy Clinic) was not given any treatment beforehand. From the use of Operationalized Adaptive Diagnostic Scale, through interviews at the beginning and at the end, diagnostics and scores were obtained to be later compared. The statistical analysis was made at 1% significance. Apart from analysis and Scores Comparison, clinical correlations inside the experimental group were evaluated. Some of them were adaptation degree, nuclear problem-situations, the different adaptation sections and the levels of improvement. The Theoretical Interpretation is the main tool employed by Operational Brief Psychotherapy, and when evaluating the efficacy of OPB, Theoretical Interpretation’s efficacy is evaluated as well. The conclusion obtained shows clear evidence that the treatment with Operational Brief Psychotherapy is effective: 86, 66% of the sample group improved their adaptive efficacy and 13, 33% remained in their adaptive group. On the control group results, 86, 66% of the sample group remained in the same adaptive group; 6, 66% decayed and 6, 66% improved