The importance of laboratory testing in the diagnosis of hereditary angioedema (HAE) has increased with the advent of new treatment options in recent years. It has been 50 years since HAE was linked to a decrease of C1INH (the inhibitor of complement enzyme, C1 esterase), a link that provided for the first laboratory test available for this disorder. HAE is subdivided into types that can be differentiated only by laboratory testing. The Type I form is characterized by low levels and function of C1INH in the circulation. The Type II form is characterized by normal levels of C1INH, but low function. Sample collection and handling is critical for the functional assays. The serum samples for the functional analysis must be collected, separated, and frozen at less than -60°C within 2 hours of the blood draw. Additionally some suspected Type II patients may benefit from looking closely at what method is used for the functional testing. The acquired forms of angioedema (AAE) can benefit from the same clinical testing, because most are ultimately due to decreased C1INH. Measurement of C1q levels and testing for anti-C1INH autoantibodies can help differentiate AAE from HAE. Diagnostic testing for the third hereditary form, alternately called estrogen-dependent HAE, HAE with Normal C1INH or HAE Type III, still presents challenges, and definitive testing may have to wait until there is a more complete understanding of this mixed group of patients. The next steps will include genetic analysis of C1INH and other proteins involved in HAE.