Familial hemophagocytic lymphohistiocytosis (FHL) is a potentially fatal disease characterized by diffuse infiltration by histiocytes and T lymphocytes. Treatment with myelotoxic drugs, such as etoposide, brings about remission in most patients, but problems of toxicity remain, and the development of disease resistance can cause secondary relapses. We have used an alternative approach, based on the suggested primary role of T-cell activation in FHL, comprising combined treatment with steroids (2 to 5 mg/kg/d methylprednisolone intravenously, followed by progressive tapering) and rabbit antithymocyte globulins (10 mg/kg/d for 5 days), followed by maintenance therapy with cyclosporine A (CSA). In a pilot study of six patients (four with a family history of FHL), all showed systemic remission within 7 days, which was complete in five cases; despite treatment with intrathecal methotrexate, one patient died of severe brain involvement. Two patients received T-cell--depleted HLA--non-identical bone marrow transplants, which was successful in one case. The other three patients, who have been on CSA maintenance therapy for periods of 6 to 24 months, are in complete remission. We have observed no side-effects (there has been no persisting T-cell immunodeficiency). These results suggest that nonmyelotoxic treatments for FHL may be safe, effective, and worthy of further investigation; they also support the key role of T lymphocytes in the disease.