Abstract Methods. The newly described - multigene analysis test (DiBiCol) identifying 7 inflammatory bowel disease (IBD)-specific genes in colonic mucosal biopsy differentiating between ulcerative colitis (UC) and Crohn's disease (CD) with active inflammation - is a new addition to existing methods with a higher stated sensitivity and specificity. Method biopsy material from 78 patients with a complicated course diagnosed as most probably UC in 38, CD in 18 and inflammatory bowel disease unclassified (IBDU) in 22 were investigated by DiBiCol. Results. DiBiCol showed a pattern consistent with CD in 13 patients with UC and led to change of diagnosis in 3 patients and a strong suggestion of CD in 8 patients. A total of 2 patients remained as UC. DiBiCol showed a pattern of UC in 4 patients of 18 with CD leading to a changing of diagnosis to UC in 3 patients, but the fourth remained as CD. In 22 patients with IBDU DiBiCol showed a pattern consistent with UC in 7 cases and with CD in 13 cases. A new evaluation 1 year after the DiBiCol allowed the assessment of clinical diagnosis in 10 patients confirmed in 9 of 10 patients by DiBiCol. In patients with acute flare of colitis the clinical diagnosis corresponded in 10 of 12 UC and in 5 of 6 CD cases. Summary. Adopting the DiBiCol test led to a change of the primary diagnosis in a significant number of patients with the initial diagnosis of UC and CD and suggested a clinically probable diagnosis in most of the patients with IBDU and in those with an acute flare of colitis.