Background: X-linked lymphoproliferative disease (XLP) is a rare primary immunodeficiency classically associated with severe Epstein–Barr virus infection. Presentation with nontuberculous mycobacterial (NTM) disease is uncommon and may obscure the underlying diagnosis. Case presentation: We report two siblings in whom the diagnosis of XLP was established following a severe infectious presentation in the index case. The first sibling presented with lifethreatening, treatment-refractory pneumonia requiring intensive care support. Microbiological investigations identified pulmonary infection due to Mycobacterium fortum(nontuberculous mycobacteria, NTM). Given the unusual severity of infection, an underlying inborn error of immunity was suspected. Comprehensive immunological evaluation followed by genetic testing confirmed a pathogenic mutation consistent with X-linked lymphoproliferative disease. Recognition of XLP in the index patient prompted targeted family screening, leading to early identification of the same mutation in a second sibling, who was asymptomatic or minimally symptomatic at the time of diagnosis. This enabled close monitoring and early immunological management before the development of severecomplications. Discussion: NTM infection is a rare but important presenting feature of XLP and should prompt evaluation for underlying primary immunodeficiency , particularly in cases of severe or atypical disease. This case highlights the critical role of early genetic diagnosis, not only for guiding management of the affected patient but also for enabling cascade screening and early intervention in at-risk family members. Conclusion: Early identification of inborn errors of immunity in children with severe NTM infection allows timely intervention and facilitates cascade screening, potentially preventing life-threatening complications in affected family members.