OBJECTIVE: To equip the reader with the tools necessary to evaluate studies of natural family planning (NFP) effectiveness found in the literature and to make recommendations for future NFP effectiveness studies. DESIGN: Current standards to evaluate contraceptive method effectiveness are reviewed. A framework for evaluating reports on NFP is presented. RESULTS: Most NFP studies found in the literature are flawed in design and do not calculate pregnancy rates correctly. The results from the few well-designed studies are presented. DISCUSSION: Many factors influence NFP effectiveness, and these factors must be considered when evaluating published studies and designing future studies.