Introduction: Idiopathic cluster of differentiation 4 (CD4+) T-cell lymphocytopenia (ICL) is a rare non human immunodeficiency virus (HIV)-related syndrome with unclear natural history and prognosis that was first reported and defined in 1992. ICL has been observed in patients after the onset of an opportunistic infection without known immunosuppression. Case presentation: A 20-year-old Hispanic male patient without significant past medical history presented with progressive shortness of breath and cough for 3 weeks. Chest computed tomography showed bilateral cavitary lesions in the upper lung lobes. The HIV rapid screening test as well as the sputum acid-fast bacilli test were both positive. The patient was started on antituberculosis therapy. The CD4 count was noticed to be low. However, the HIV Western blot test was negative, and the HIV viral load was within normal limit. Further radiologic studies, hemato-oncologic, and autoimmune workups were normal. The patient was discharged on the treatment for tuberculosis. Follow-up after 8 weeks revealed a persistent low CD4+ count, and the repeated HIV tests were negative. Conclusion: The clinical features of ICL range from an asymptomatic condition to life-threatening complications that imitate the clinical course of HIV-infected patients. The differential diagnosis in adults comprises primarily HIV infection and other diseases or drug side effects. ICL is very rare and should be considered in the absence of any defined immunodeficiency or therapy associated with depressed levels of CD4+ T-cells. Early detection and recognition of the disease allow purposeful and systemic treatment approach and screening for the affected patients.