Making a correct diagnosis of a primary immunodeficiency disease is crucial for the selection of proper therapy. Although many cases go undiagnosed, there are also many instances of incorrect diagnosis that result in years of inappropriate treatment and failure to implement beneficial treatment. This article summarizes 2 actual cases in which incorrect diagnoses led to recommendations of unwarranted high-risk or costly treatments. Had the physicians chosen tests of immune function rather than relying on immunoglobulin levels or cell counts, they would have arrived at the true diagnoses.