Many non-invasive tests have been studied for diagnosis and determining the activation degree of inflammatory bowel disease (IBD). Nevertheless, an ideal test has not been found yet. Mean platelet volume (MPV) is influenced by the inflammation. In a few study, decreased platelet volume have been reported in IBD. The aim of this study is to determine whether platelet volume would be useful in ulcerative colitis (UC) activity. Additionally we have analyzed overall accuracy of MPV in disease activity and compared with other inflammatory markers. A total of 61 UC patients (male/female : 41/20), and 27 healthy subjects (male/female : 18/9) were enrolled into the study. For all subjects following tests were performed; ESR, CRP, white blood cell count and mean platelet volume. A statistically significant decrease in MPV was noted in patients with UC (8.29 +/- 1.02 fL) compared with healthy controls (8.65 +/- 0.79 fL). MPV of active UC (8.06 +/- 1.19 fL) patients were significantly lower than that of inactive UC (8.45 +/- 0.87 fL). Overall accuracy of MPV in determination of active UC was 71% (with sensitivity 67%, specificity 73%). A negative correlation was found between MPV and endoscopic activity index (r : -0.358 p : 0.005). In UC, MPV did not correlate with ESR, CRP and white blood cell. Our study showed that MPV reduced in UC, particularly in patients with active UC. Decreased MPV may be an indicator for increased disease activity in patients with UC.