Objective: To review the effectiveness of statewide newborn screening for cystic fibrosis (CF) in Victoria over the first 10 years of the program (1989-1998)., Design: Population study involving screening of newborns by immunoreactive trypsinogen (IRT) testing on Day 3-5, followed by either repeat IRT testing (1989-1990) or delta F508 mutation analysis (1991-1998)., Patients and Setting: All babies screened for CF in a newborn screening program in Victoria in 1989-1998., Main Outcome Measure: The diagnosis of CF., Results: Of 635,157 babies born in Victoria in the 10 years, 191 were diagnosed with CF. A further 30 cases were detected antenatally, giving an incidence of 1/2874 (95% CI, 1/2519-1/3294). CF was detected early in 182 babies (95.3% of affected babies in the screened cohort)--136 by screening, 35 because they had meconium ileus, and 11 because they were siblings of older children with CF. Nine cases of CF were missed by screening. Of these nine babies, four did not have an elevated neonatal IRT level, one had a normal IRT level at repeat testing at 4-6 weeks (1989-1990), three did not have a delta F508 mutation (1991-1998), and one had a false negative sweat test result. Six of the nine missed babies (67%) were diagnosed within four months of birth., Conclusion: Newborn screening for CF in Victoria has proven effective in detecting most babies with CF in the newborn period. However, a sweat test should be requested when the clinical features suggest the diagnosis of CF, even if the child has been screened.