The parents of one hundred families (fifty in Scotland and fifty in Northern Ireland) affected by cystic fibrosis were interviewed. It was found that their understanding of the genetics of the disease was unsatisfactory. Poor education and low intellectual ability were factors, but the principal causes were psychological barriers to understanding in the parents and inadequate and badly timed instruction by the diagnosing physician. Lack of understanding contributed to the unhappiness of these families, in which the incidence of guilt, anxiety, and depression was high, especially among the mothers. In both countries over 70% of the parents wished to have no further children following the diagnosis of cystic fibrosis in one of their children. In Ireland the principal reason was fear of conceiving a further affected child. This was the reason given by only 36% of couples in Scotland, where the size of family already reached was an equally important consideration in deciding to limit the family. In those families in which a decision had been made to have no further children, the incidence of unintended pregnancies was high. Although only one of these pregnancies resulted in a child suffering from cystic fibrosis, more active steps should be taken to provide adequate family-planning advice for the parents of cystic-fibrosis patients.