Pancreatic insufficiency occurs in the majority of cystic fibrosis (CF) patients. Deficient fluid secretion is apparent at all levels of pancreatic function and leads to pancreatic protein hypersecretion which may in turn result in protein precipitation and ductal plugging. An impaired chloride and bicarbonate secretion appears to account for this fluid secretion deficit. A minority of CF patients have sufficient preservation of pancreatic function to prevent steatorrhoea. These patients are diagnosed at a later age, experience milder symptoms and have a far superior overall prognosis than patients with pancreatic insufficiency (PI). Patients who are homozygous for delta F508 have a high frequency of PI (99%), whereas patients with other genotypes are more often pancreatic sufficient (PS). In 538 patients with CF DNA analysis was performed and related with pancreatic function. The most striking observation was that nearly all given genotypes were associated with either PI or PS and not with both. In addition, we were able to classify mutations as "severe" and "mild" with respect to pancreatic function.