Plasma platelet factor 4 (PF4), secreted by the platelets, is an index of platelet aggregation and thromboembolic risk. The authors assessed PF4 in 20 patients with Crohn's disease (ileitis in 13 patients, ileocolitis in seven) and in 20 healthy volunteers. Disease activity was low (Crohn's Disease Activity Index less than 150) in 11 patients and high in nine. Radioimmunoassay of PF4 using Abbott's Kit was performed on one sample of plasma from each subject (nv less than or equal to 0.324 nmol/ml), (nv less than or equal to 10 ng/ml). A significantly higher concentration of PF4 was found in Crohn's disease patients: 4.625 +/- 1.1 nmol/ml (142.5 +/- 36 ng/ml) than in the control group: 0.189 +/- 0.07 nmol/ml (5.6 +/- 4.8 ng/ml) (Z = 5.396, p less than 0.0001). No correlation was present between PF4 levels and activity, the site of disease, or medical treatment with or without prednisone.