Objective: In active stages of ulcerative colitis (UC), a tendency for neutrophils to aggregate in the colonic lamina propria is mediated by yet unidentified surface receptors. The aim was to assess the spontaneous leukocyte aggregation and the aggregation induced by bacteria-derived products in UC and to evaluate the involvement of ICAM-1 and β₂-integrins in this aggregation. Materials and methods: Blood was drawn from 10 patients with quiescent UC, 10 patients with active UC, and 10 healthy volunteers. The blood was stimulated with LPS or fMLP with subsequent blocking of CD11b or ICAM-1 with specific antibodies. The aggregation was assessed on glass slides with an automated image analyzer (Inflamet™). Results: The spontaneous leukocyte aggregation was increased in quiescent and active UC as compared to healthy controls (p < 0.05). Although not statistically significant, LPS and fMLP seemed to increase the leukocyte adhesiveness, and also a tendency towards inhibition of the leukocyte aggregation was observed by blocking ICAM-1. Conclusions: Increased adhesiveness of circulating leukocytes seems to be involved in the pathogenesis of UC, and ICAM-1 is suggested to be a part of this phenomenon. The results indicate an altered basic neutrophil response in UC.