An improved understanding of the expression of the cystic fibrosis gene (CFTR) will assist our approach to preventing the organ damage caused by cystic fibrosis (CF). We have studied the expression of CFTR in human fetal tissues at different gestational ages using in situ hybridization to detect CFTR mRNA. CFTR was principally expressed in less differentiated cells of endodermal origin. The highest levels were seen in specific areas of the developing pancreas, liver, gall bladder and intestine, with lower but significant levels in lung and trachea. Expression was also seen in reproductive tissues, such as epididymis and third trimester uterus and fallopian tubes, and in addition, sweat and salivary glands. No detection of CFTR mRNA was found in many other relevant tissues. The detection of CFTR transcript in these organs is consistent with the clinical manifestations of CF and the function of CFTR as a chloride channel early in development. The localization and levels of expression described have implications regarding the pathogenesis of organ damage and the potential gains that can be achieved by early therapy in the disease.