Cystic fibrosis (CF), is the most common autosomal-recessive disease in Caucasians, with an incidence of approximately 1:2500 live births and a carrier frequency of approximately 4-5%. Causes of the disease are mutations in the CF gene which is located on chromosome 7 (region 7q31). Although a single mutation, a deletion of phenylalanine at position 508 (DF508) in exon 10, accounts for almost 70% of all CF chromosomes, over 900 other mutations have been identified in this large gene. CF gene encodes a membrane protein, which functions as aion channel- CFTR (cystic fibrosis transmembrane regulator protein). The exocrine pancreas is a gland that secretes water, enzymes and electrolytes into the intestinal lumen. These enzymes are needed for the normal digestion of food, and their reduced secretion in cystic fibrosis will cause malabsortion and malnutrition in CF patients. Pancreatic dysfunction in CF begins in uteri. Most patients with CF typically present insufficient pancreatic exocrine function (PI phenotype) and 10-15% of CF patients are pancreatic sufficient (PS phenotype). It has been shown elsewhere that the pancreatic function status in CF could be correlated to mutations in the CFTR gene. To determine the relation between genotype and pancreatic status, we analyzed 32 CF patients in whom both CF gene mutant alleles were identified (Table 1). Patients included in this study attended the Paediatric Department of Mother and Child Health Institute in Belgrade. The diagnosis was based on typical clinical manifestations and high levels of sweat chloride concentration (higher than 60 mmol/L). Of the 32 patients studied, only one (3.12%) was PS and the rest (96.88%) had PI phenotype. For each CF genotype the number of patients who were PI or PS is given in Table 1. The most striking observation was that all given genotypes correlated with either PI or PS, but not with both. On the basis of both preceding hypotheses and our present data (Table 2 and Table 3), it was possible to classify mutations as "severe" or "mild" with respect to pancreatic function (Table 4). This study strengthens the hypothesis that pancreatic function status in CF is genetically determined by specific mutations at the CF locus. Our data also strongly support the hypothesis that, with respect to pancreatic function, "mild" mutant alleles confer a higher residual CFTR activity than do "severe" mutant alleles. Although PS occurs in patients who have one or two "mild" mutations, PI occurs in patients who are homozygous or who are genetic compounds of two "severe" mutant alleles.