Cystic fibrosis (CF) is the most common severe inherited disorder that affects children in Caucasian populations. The aim of this study was to define the spatial and temporal distribution of CF and its mutations in Brittany (western France) where the frequency of the disease is high. We retrospectively registered all CF patients born in Brittany since 1960 by cross-checking various data sources (e.g. medical care centres, genetics laboratories, hospital archives). Councils were contacted so that the place of residence of patients at birth could be determined. Moreover, the spectrum of CF transmembrane conductance regulator (CFTR) mutations and their spatial distribution across Brittany were determined. A total of 520 patients was registered in this study. The incidence of CF was assessed according to administrative (department, district) and diocesan divisions of Brittany and its evolution analysed over four decades. The incidence of CF was 1/2630, with a west/east gradient that was confirmed over time (Finistère: 1/2071 vs Ille-et-Vilaine: 1/3286). At present, the incidence of CF is decreasing, mainly as a result of prenatal diagnosis. An excellent mutation detection rate of 99.7% was obtained. Western Brittany presented a specific spectrum of mutations: 1078delT (9.4% of mutated alleles in the diocese of Cornouaille), G551D (7.7% in the diocese of Léon), 4005+1G-->A (2.9% in Cornouaille) and W846X (1.5% in western Brittany). On the other hand, the eastern region showed a spectrum more similar to the overall picture in France as a whole. This study enabled a precise measurement of the incidence of CF in Brittany to be obtained. The high frequency of the CFTR mutated alleles may result from founder effects and genetic drifts. Moreover, the study brings together the regional specificities of the CFTR gene and highlights disparities that exist in this part of France, both in incidence and in mutation distribution. These are attributable to different degrees of isolation and of population movements between the eastern and western parts of the region. Given that this is the first time that such a detailed study of the CFTR gene has been performed on a large population, this heightened knowledge of the epidemiology of CF in Brittany should provide a basis for the improvement of diagnostic strategies and refinement of genetic counselling.