Background: Apoptosis inhibitor of macrophages (AIM) was initially identified as an apoptosis inhibitor that supports the survival of macrophages against various apoptosis-inducing stimuli, and AIM produced by macrophages may contribute to the pathogenesis of inflammatory bowel diseases (IBDs). However, there have been no reports on the kinetics of AIM in IBD and the impact of AIM on the pathogenesis of IBD. In this study, we aimed to investigate the diagnostic utility of levels of AIM and their correlation with the activity of Crohn's disease (CD) and IBD. Methods: We used an enzyme-linked immunosorbent assay (ELISA) to examine AIM serum levels in 16 healthy subjects and 90 patients with inflammatory bowel diseases, namely 39 with CD and 51 with ulcerative colitis (UC), as well as 17 patients with Behcet's disease (BD) as intestinal disease controls. We compared serum AIM levels among groups and examined whether there were correlations between serum AIM levels and disease activity and type. We also performed immunohistochemical staining of AIM in intestinal tissues of patients with CD. Results: Serum AIM levels were significantly higher in patients with CD than in patients with UC, BD, and controls (3.27 +/- 2.14, 1.88 +/- 1.43, 2.34 +/- 1.37, and 2.13 +/- 0.64 mu g/ml, respectively; P < 0.01). There was no difference in serum AIM levels before and after treatment in patients with CD. However, in these patients the diagnostic rate using AIM was better than that based on anti-Saccharomyces cerevisiae antibodies. AIM was expressed in macrophages that were positive for CD14, CD16, or both in the intestinal tissues of patients with CD. Conclusions: AIM is a novel biomarker of CD that can distinguish CD from UC or BD. It is suggested that AIM may contribute to intestinal inflammation by inhibiting the apoptosis of macrophages.