• We examined the accuracy of laboratory screening tests in eight neonatal intensive care units. Analytes were provided by the EXCEL proficiency testing program of the College of Ameri American Pathologists (CAP), Skokie, III, samples were analyzed for hematocrit values and urine specific gravity. "Dipstick" tests were also performed for urinary protein, bilirubin, blood, glucose, pH, and ketones. The one intensive care unit that adhered to a formal quality control program for all these tests uniformly reported results well within statistically acceptable limits as defined by CAP. The seven other centers reported 14 instances of results that were either greater than 1 SD OF THE mean or considered by the consensus method to be outside acceptable CAP limits. Three of these results may have led to inappropriate clinical action in neonates. We conclude that there is a real potential for errors to be made in screening tests run by nurses in a neonatal intensive care unit when a formalized quality control program is not in effect.