We report on two patients with idiopathic CD4+ T cell depletion. A 26 year-old woman presented to us with acute respiratory failure requiring mechanical ventilation. Despite combined antibiotic therapy parenterally the opacities increased in the chest X-ray. An open lung biopsy was performed and led to the histological diagnosis of bronchiolitis obliterans organizing pneumonia (BOOP). Respiratory function was improved impressively by high dose parenteral cortisone administration. This patient showed a general lymphocytopenia with severe CD4+ T cell depletion (60(37%)/mm3 blood). The CT4+ T cell concentration increased during a follow up period of 14 months, but did not reach normal values. The second patient was a 33 year-old homosexual. He was admitted with a mucocutaneous fungal infection which was successfully treated by antifungal drugs. This patient demonstrated a transient CD4+ T cell depletion (350(32%)/mm3 blood). In both patients HIV type 1 and 2 infections were excluded by antibody- and p 24-antigen testing, polymerase chain reaction and virus culture. CONCLUSION. Idiopathic CD4+ T lymphocytopenia differs from HIV infection in immunological profile, in the tendency to reversal of the CD4+ T cell depletion over time and in its better prognosis. It is unclear if this is a new syndrome and whether a transmissible agent, or possibly a genetically-determined reaction to noxious agents is responsible.