Many questions remain regarding the efficacy, risks, and costs of CF neonatal screening. The major gap in knowledge that must be closed before CF neonatal screening can be recommended generally in the United States concerns the potential long-term medical benefits of initiating treatment in early infancy. It would be premature, in our opinion, to implement mass population screening of newborns for CF until the benefits and risks have been fully defined, and an adequate and logistically feasible testing system developed and/or highly effective therapy for CF lung disease becomes available. It is for this reason we designed a randomized, controlled investigation of CF neonatal screening and implemented this project in Wisconsin during 1985. The fact that 5 years of randomized screening and systematic evaluation of outcome measures have not yet revealed any pulmonary benefits underscores the importance of rigorous investigation to resolve the efficacy issue. In addition to the medical uncertainties, we believe that the ethical issues described herein need to be resolved; this concern pertains not only to the CF patient but also the heterozygote carrier. On the other hand, financial factors and uncertainty about the cost effectiveness of CF neonatal screening do not appear to be dominant issues according to our assessment of current data. Despite the reservations related to the benefit/risk relationship, we expect that the discovery of the CF gene should have a favorable impact on neonatal screening for the disease, as well as for management.