Inborn errors of immunity with immune dysregulation, many of which belonging to the subclass of combined immunodeficiencies, often present with variable severity even within one family. Predictors of the clinical course are difficult to identify, and the benefit-risk ratio of hematopoietic stem cell transplantation needs to be weighed against the risks of a chronic disease burden and lifelong immunosuppressive therapy. Therefore, we aimed to develop a disease activity score using a retrospecitvely collected dataset of patients with LRBA deficiency. The score was named IDDA score and aims to take into account all known phenotypical features of this severely compromising inborn error of immunity with autoimmunity, grading their severity, and it is multiplied by physician-reported outcome measures such as hospitalization days, requirement and intensity of supportive care, and performance scores. The usefulness of the IDDA score to monitor disease activity longitudinally, and ideally, also cross-sectionally (intra- and interindividually, respectively), and to improve the risk-benefit assessment of various treatment options in LRBA deficiency is being calculated and will be open for discussion. If successful, the IDDA score might be applied in patients with similar disorders to facilitate their management.