Background. Several serologic tests, including anti-outer membrane porin C antibody (Omp C), are used for screening and as marker of disease course in inflammatory bowel diseases (IBD). Our aim was to investigate possible differences in Omp C level in patients with active and inactive IBD compared to controls. Methods. All blood samples were tested for Omp C. Disease activity was evaluated by Harvey Bradshaw Index, Simple Clinical Activity Index and Modified Pouchitis Disease Activity Index. Results. Blood samples were collected from 113 patients and 60 controls. Patients with active IBD did not have a higher level of Omp C than patients in remission. Surprisingly, in patients with active Crohn's disease a significantly lower level of Omp C was found compared with patients with inactive Crohn's disease (p < 0.05). All other groups among patients with IBD did have a significantly higher level of Omp C, compared with controls, including patients with acute gastroenteritis (p < 0.05). Although IBD patients with phylogroup B2 E. coli cultured from their fecal samples, were more likely to have a positive Omp C test (p < 0.05), this could not explain the low Omp C level in the subgroup of patients with active Crohn's disease. Conclusions. Omp C titer was not raised in patients with active IBD compared with patients in remission. In addition, there was no difference in Omp C level in patients with active Crohn's disease compared with controls. These observations do not support the use of Omp C serology testing, either in disease activity assessment, or in screening for active Crohn's disease.