In a sruvey of 119 cystic fibrosis (CF) centres in the U.S.A. and Canada, information was obtained on 129 pregnancies in 100 patients with CF. 97 pregnancies were completed resulting in 86 viable infants, only one of whom had CF. This is close to the expected incidence and attests to the recessive nature of the transmission of CF, the commonest lethal genetic disorder of Caucasians. Shortened gestation and increased maternal and perinatal mortality were related to severe maternal pulmonary infection. Increased respiratory and cardiac work, and especially hypervolaemia, are hazards for pregnant women with CF, and when combined with advanced pulmonary disease may lead to increased perinatal complications. Maternal hypoxia and transplacental passage of antibiotics may affect the fetus; however, there were no congenital anomalies in this series in spite of frequent use of antibiotics by these mothers. Unless the clinical score is high, pregnancy should be avoided in CF patients.