Reactive thrombocytosis is a typical feature in inflammatory bowel disease (IBD). The question arose as to whether the normal negative feedback regulation of the concentration of thrombopoietin (TPO) in blood was altered in IBD patients. We measured serum immunoreactive TPO in 30 patients with active IBD, 29 patients with inactive IBD, and 56 healthy controls. The results were related to platelet and leukocyte counts and to the serum concentration of interleukin 6 (IL-6). Patients with active IBD exhibited significantly increased TPO levels (medians 112 pg/ml vs. 90 pg/ml in controls, p < 0.05) in association with thrombocytosis (428 platelets/nl blood vs. 241 platelets/nl blood in controls), leukocytosis, and increased IL-6 levels (12.9 pg/ml vs. 2.5 pg/nl in controls). In patients with inactive IBD, only platelets (322/nl) and leukocytes were above normal. Although the observation of increased TPO and IL-6 levels provides an explanation for the occurrence of thrombocytosis in IBD, the pathogenetic mechanisms underlying the elevated TPO level still need to be identified.