A 55-year-old woman who had been suffering from ulcerative colitis since 1973 was admitted to our service in 1999 for chronic thrombocytosis discovered 9 months previously (600 giga/L). The only treatment prescribed was sulfasalazine. In 1988, a hysterectomy was performed without any blood coagulation event. Clinical examination was normal. Biological evaluation showed an isolated high platelet count of 464 giga/L with Howell-Jolly bodies, without either inflammatory process or iron deficiency. She presented a normal thromboplastin time and a prolonged TT of 77 sec (control: 26 sec) with human thrombin. Reptilase time was normal. TT was not corrected by mixing with an equal volume of normal plasma, which evoked the presence of an acquired anti-thrombin inhibitor. Fibrin level was normal. Platelet function tests showed only early ADP-disaggregation. No anticardiolipin, anti-ß2-glycoprotein 1 antibody, lupus anticoagulant, antinuclear, or anti-neutrophil cytoplasm antibodies were found. Antigliadin and endomysium antibodies were negative. Immunoglobulin level was normal without monoclonal gammopathy. Bone marrow biopsy and thorax CT-scan were normal. Abdominal CT-scan revealed a spleen of reduced size. We concluded there was thrombocytosis related to hyposplenism associated with ulcerative colitis and ATa. The prolonged TT with otherwise normal coagulation parameters may be caused by presence of heparin, dysfibrinogenemia, paraproteinemia, or disseminated intravascular coagulation. All of these etiologies were excluded in our patient.