ABSTRACT: BACKGROUND: Cystic fibrosis (CF) is an autosomal recessive disorder whose incidence has long been estimated as 1/2500 live births in Caucasians. Expanding implementation of newborn screening (NBS) programs now allows a better monitoring of the disease incidence, what is essential to make reliable predictions for disease management. This study assessed time trends in the birth incidence of CF over a long period (35 years: 1975-2009) in an area where CF is frequent (Brittany, France) and where NBS has been implemented for more than 20 years. METHODS: This study enrolled CF patients born in Brittany between January 1st 1975 and December 31st 2009 (n=483). Time trends in incidence were examined using Poisson regression and mainly expressed using the average percent change (APC). RESULTS: The average number of patients born each year declined from 18.6 in the late 1970's (period 1975-79) to 11.6 nowadays (period 2005-09). The corresponding incidence rates dropped from 1/1983 to 1/3268, which represented a decline close to 40% between these two periods (APC=-39.3%, 95% CI=-55.8% to -16.7%, p=0.0020). A clear breakpoint in incidence rate was observed at the end of the 1980's (p<0.0001). However, the incidence rate has remained quite stable since that time (annual APC=-1.0%, 95% CI=-3.0% to 1.1%, p=0.3516). CONCLUSIONS: This study provides an accurate picture of the evolution of the incidence of a genetic disease over a long period and highlights how it is influenced by the health policies implemented. We observed a 40% drop in incidence in our area which seems consecutive to the availability of prenatal diagnosis.