In the paper results were presented of a study on manifestation of infection with Mycobacterium avium subsp.paratuberculosis in 16 patients aged 15-42 years with Leśniowski-Crohn disease (group 1), in 20 patients aged 21-50 years with ulcerative colitis (group 2) and in 12 healthy individuals aged 23-60 years (group 3, control). All the ill patients were subjected to surgery, involving partial or total resection of large intestine, while individuals in group 3 (control) were subjected to colonoscopy with sampling of large intestine. Using mechanical/enzymatic technique DNA was extracted from the tissue material and was identified using PCR-ELISA technique (Mycobacterium paratuberculosis PCR; Institut Pourquier-France). Colour reaction was evoked using the TMB substrate. In the studies presence of Mycobacterium avium subsp. paratuberculosis was noted in 10 (62.5%) patients with Leśniowski-Crohn disease, in 5 (25%) patients with ulcerative colitis and in 1 patient 1 (8.3%) patient of the control group. The obtained results permit to suggest that Mycobacterium avium subsp. paratuberculosis bacteria participate in etiopathogenesis of Leśniowski-Crohn disease.