ICOS co-stimulation of peripheral blood T cells for 48 or 72 hours significantly enhanced expression of ICOS on CD3+CD45RO+ T cells in both patients with inactive SLE and normal control individuals (patients with inactive SLE: 12.6 ± 3.9% before stimulation versus 27.5 ± 18.7% 48 hours after stimulation versus 63.5 ± 3.3 % 72 hours after stimulation; normal control individuals: 33.6 ± 28.0% before stimulation versus 53.2 ± 26.9% 48 hours after stimulation versus 67.2 ± 29.3% 72 hours after stimulation; P < 0.05 for both 48 and 72 hours compared with before stimulation in each group). The gene discussed is ICOS; the disease is systemic lupus erythematosus.