The review article deals with pathogenesis and diagnostics of common gastrointestinal (GI) complications in patients with oncohematological diseases subjected to intensive chemotherapy and allogeneic hematopoietic stem cell transplantation (allo-HSCT). The associated GI disorders may present with hyporexia, nausea, vomiting, diarrhea, dehydration and intoxication thus causing protein-energy malnutrition and reducing efficiency of cancer therapeutics. The symptoms of intestinal dysfunction may develop early before HSCT due to cytostatic and antibacterial therapy. As a rule, it manifests with diarrhea caused by different reasons, e.g., antibiotic-associated bacterial dysbiosis, or enteropathogenic infections, impaired intestinal motility, or due to immune-mediated graft-versus-host disease (GvHD).