Ulcerative colitis (colitis ulcerosa, CU) and Crohn’s disease belong to a group of nonspecific inflammatory bowel diseases characterized by chronicity and unknown aetiology. Due to their heterogeneous pathogeneses, their clinical pictures and clinical courses vary. This results in diagnostic difficulties that are encountered despite the availability of a number of specialized tests in contemporary medicine. It is known that a condition necessary to establish a definitive diagnosis is to confirm the presence of a nonspecific inflammatory bowel disease in a histopathological examination, but laboratory tests are helpful in the initial diagnosis and demonstrate the extent and severity of the inflammatory process. C-reactive protein is the indicator of the inflammatory process, and anaemia that occurs in ulcerative colitis is due to both the chronicity of the disease and iron deficiency secondary to blood loss. Laboratory diagnosis is also applicable in the evaluation of parenteral complications concerning the liver, bile ducts, and osteoporosis. Abnormal liver function tests are frequent in inflammatory bowel diseases and may be caused by an ongoing inflammatory process typical of these diseases as well as by the pharmacological treatment implemented. Although new methods, such as computed tomography, magnetic resonance imaging and positron emission tomography, are used in diagnosing gastrointestinal diseases, endoscopy with histopathological analysis of the collected samples still is (and will be for a long time) a primary diagnostic method in the process of diagnosing, monitoring the course of treatment and oncological surveillance.