In order to examine the onset of the cystic fibrosis (CF) disease process, the expression of the cystic fibrosis gene (CFTR) has been examined in mid-trimester human foetal tissues by in situ hybridization. CFTR mRNA was detected in the epithelia of pancreatic ducts, small intestine, colon, genital ducts, lung and trachea. The majority of these sites of CFTR expression in the foetus are similar to those seen in adult tissues. However, epithelia of the lung, that contain very little CFTR mRNA in the adult, express high levels of CFTR mRNA in the foetus. Since the lung is the major site of pathology and morbidity in CF these findings have implications for treatment.