Although routine analysis of the complement system is rarely demanded, it is important from a diagnostical and therapeutical point of view to include the relevant tests when clinically required. Complement analysis should be performed when hereditary angioedema (C1 inhibitor deficiency) is suspected, and as part of investigation of patients with immunodeficiency (recurrent infections in childhood). Furthermore, complement analysis may be required in cases of recurrent systemic neisserial infections at any age, and to follow up certain autoimmune diseases. Only a few tests are available for routine purposes. These are reviewed briefly, with emphasis on collection and storage of samples prior to analysis. It is important that these guidelines be followed, to avoid misinterpretation of the results.