In recent years the ability to detect cystic fibrosis (CF) mutations has greatly expanded the clinical spectrum of the disease. In a rising number of patients an atypical, usually mild phenotype is found; in others a single clinical feature predominates. A diagnosis of CF is usually formulated following a positive sweat test, or the identification of disease-causing gene mutations, or the in vivo demonstration of the typical abnormalities in ion transport across the nasal or rectal epithelium. However, these same tests, although helpful, may prove inconclusive in individuals with such an unusual clinical presentation. Very little information is available about the possible long-term evolution of these non classical forms of disease.