Introduction Though based firmly on the "old fashioned" disease of phenylketonuria, whole population blood based neonatal screening is a technique with many potential uses. Most never enter general service despite their technical feasibility, reflecting mainly the perceived balance between costs and benefits. For historical reasons there is an underlying assumption that neonatal screening should make a "profit".' Hitherto, such considerations have not been widely applied to other branches of medicine, but with the NHS reforms this is changing. A broader perspective on cost effectiveness, together with advances in treatment and the increased importance being attached to genetic information, may lead to greater enthusiasm for screening. New tests are still being developed and recent instrumental advances may make it practicable to screen for groups of disorders where this has previously been too difficult or too time consuming.