BACKGROUND/AIMS: Cystic fibrosis (CF) in infancy and childhood is often associated with failure to thrive (FTT). This would suggest that in countries without a newborn screening program for CF, FTT could be used as a clinical screening tool. The aim of this study is to assess the diagnostic performance of FTT for identifying children with CF. METHODS: Longitudinal length and weight measurements up to 2.5 years of age were used from CF patients (n = 123) and a reference group (n = 2,151) in The Netherlands. Growth measurements after diagnosis were excluded. We developed five potential screening rules based upon length, weight and body mass index (BMI) standardized by age and gender (SDS). Outcome measures were sensitivity, specificity and positive predictive value (PPV). RESULTS: BMI SDS had the highest sensitivity at low false-positive rates. An efficient scenario is a BMI SDS below -2.5 SD in combination with a decrease in BMI SDS of at least 0.5 SD. This scenario had a sensitivity of 32%, a specificity of 98.3% and a PPV of 0.75%. CONCLUSION: In the absence of a newborn screening program, young children with FTT for BMI are candidates to consider testing for CF.