Interest in natural killer (NK) cell analysis has primarily been for patients with otherwise unexplained reproductive failure. Much of the controversy surrounding NK cell analysis is largely the result of poor study design, over-interpretation of results, and lack of understanding of the complexities of the laboratory methods used. The assessment of uterine (u) NK cells is normally done by immunohistochemistry, the subjectiveness and limitations of which are rarely appreciated. Considerable care is essential to assess the methods of NK analysis, and treatment protocols are highly variable, often including multiple therapy with intravenous immunoglobulin (IVIG), aspirin, heparin, and dexamethasone. Given these constraints though, it has been shown that both uNK and blood (b) NK cell numbers and activity can be suppressed by IVIG and by prednisolone. It has also been shown that in women with repeated IVF failure with high NK cell activity, treatment produces significantly better pregnancy rates.