Hereditary angioedema (HAE), resulting from inherited deficiency of C1 inhibitor, is a rare but disabling and life-threatening disease. An overshooting local increase in bradykinin concentration induces angioedema attacks, which manifest as acute abdominal pain, and facial, laryngeal or peripheral swellings. Health care providers are often not aware of appropriate differential diagnosis and therapy of this rare disease, which frequently bears resemblance to other disorders. For this reason we convened the working group of the HAE Section members experienced in HAE management to develop recommendations for Polish clinical decision makers. The document was initiated at the working meeting and then circulated for further suggestions. The independent review board was requested to review the document and provide feedback. This first part of the consensus statement covers clinical symptoms, differential diagnosis, classification, and pathophysiologic background of HAE, including new genetic findings.