Flow cytometry analysis of in vitro activated basophils (BATs) based on the detection of CD63 up-regulation on basophil membrane provides the physician and the clinical laboratory with a novel diagnostic approach, proposed as a promising alternative method for in vitro diagnosis of IgE and non-mediated reactions. We performed an optimized flow cytometric procedure to assess CD63 expression on activated basophils on twenty allergic patients, and compared the results with specific IgE determination (RAST) and skin testing (Prick test).