Serum levels of soluble interleukin-2 receptor were determined in 29 patients with active and quiescent Crohn's disease. In addition, the ability of peripheral blood mononuclear cells of 23 of these patients to generate soluble interleukin-2 receptor following mitogenic stimulation was studied in vitro. Serum soluble interleukin-2 receptor concentrations of patients with active Crohn's disease (n = 19) were significantly elevated (757 +/- 438 U/ml) compared with levels in patients with inactive disease (n = 10; 412 +/- 120 U/ml) and healthy control individuals (n = 40; 375 +/- 102 U/ml; p less than 0.003 and p less than 0.0005, respectively). Serial determinations of serum soluble interleukin-2 receptor concentration in a follow-up of 11 hospitalized patients treated for highly active disease showed a decrease from 1252 +/- 494 U/ml to 527 +/- 193 U/ml (p less than 0.004) that corresponded to clinical improvement, as assessed by Crohn's disease activity index and a reduction of inflammatory parameters. In vitro phytohemagglutinin stimulation of peripheral blood mononuclear cells derived from patients with Crohn's disease resulted in elevated soluble interleukin-2 receptor production not only in patients with active disease (3987 +/- 2439 U/ml), but also in patients with inactive disease (3297 +/- 2282 U/ml), compared with the amount of soluble interleukin-2 receptor produced by mononuclear cells of healthy individuals (1523 +/- 1152 U/ml; p less than 0.005 and p less than 0.02, respectively). In addition, cultivation of mononuclear cells without mitogen resulted in higher soluble interleukin-2 receptor production in patients with active disease than in patients with inactive disease (p less than 0.02). However, patients suffering from active ulcerative colitis also had significantly increased serum levels of soluble interleukin-2 receptor (1080 +/- 400 U/ml) compared with the levels in patients with chronic disease (455 +/- 140 U/ml; p less than 0.0025). In addition, peripheral blood mononuclear cells derived from patients with ulcerative colitis produced significantly more soluble interleukin-2 receptor upon mitogenic stimulation with phytohemagglutinin (2314 +/- 936 U/ml), than cells from healthy controls (1523 +/- 1152 U/ml; p less than 0.05). The finding of elevated soluble interleukin-2 receptor serum levels in patients with active Crohn's disease and its increased production by mononuclear cells of patients with both active and inactive disease is a further example of an alteration of the immune system in this condition; however, this alteration can also be found in other inflammatory bowel diseases.