In addition to systemic manifestations with skeletal, pulmonary, renal, and haematological signs, lysinuric protein intolerance (LPI), a membrane transport defect of cationic amino acids, is often complicated by severe life-threatening immunological manifestations. A 10-year-old boy with LPI who exhibited a severe systemic immunohaematological disease is described here. This patient showed cutaneous lesions similar to the subacute form of systemic lupus erythematosus, severe anaemia and dysproteinaemia, and a marked reduction of circulating T lymphocytes, mainly the CD4+ cells. In vitro bone marrow cell culture studies showed that addition of patient's serum induced macrophage proliferation and inhibited erythroid progenitor cell growth. Treatment with high-dose intravenous immune globulin resolved most of the clinical and laboratory abnormalities.