Previously we have reported that 37% of infants with cystic fibrosis diagnosed by neonatal screening with the dried blood spot immunoreactive trypsin assay have pancreatic sufficiency. However, 34 of the 78 infants had pancreatic function tests an average 2.3 years after diagnosis, thus it was possible that the percentage with neonatal pancreatic sufficiency was underestimated, due to the loss of pancreatic function with time in some infants. To assess this hypothesis we have assessed pancreatic function at the time of diagnosis in a further 20 infants since the completion of the previous study. Results of fecal fat determinations and/or pancreatic stimulation tests indicate that 10 (50%) of these infants have pancreatic sufficiency. Combining these results with those of the previous study, 31 of 64 patients (48%) have pancreatic sufficiency at this early age. We have also monitored the progression of pancreatic disease in the 39 patients with pancreatic sufficiency recognized to date. Eleven have developed pancreatic insufficiency and require enzyme replacement therapy. Five others have shown further improvement of colipase secretion with age. We conclude that the dried blood immunoreactive trypsin screening program for cystic fibrosis does recognize patients with pancreatic sufficiency, and at diagnosis nearly half the patients are in this category. To date, 28% of patients with pancreatic sufficiency have demonstrated a variable decline in pancreatic function with age.