We aimed to discover whether neonatal infection affected levels of the immunoreactive trypsin (IRT) assay, which is used as a screening test for cystic fibrosis. Forty babies who had clinical features suggesting infection had blood spot specimens for IRT taken at the same time as blood for culture. Of 27 babies believed clinically to have infection, 19 had positive blood cultures and eight had negative culture. In retrospect, 13 were not thought to have infection. The mean (SD) IRT results in these three groups were 15 (8), 11 (4), and 16 (6) micrograms/L, respectively. The mean (SD; range) IRT result for all 40 babies was 14 (7; 10-39). Not only did no babies have elevated IRT results at routine neonatal screening, but none have presented with clinical features suggestive of cystic fibrosis since hospital discharge. Neonatal infection does not appear to cause elevated IRT levels and should therefore not cause false-positive results in mass neonatal screening.