Now that the cystic fibrosis (CF) gene has been identified, direct genetic testing for this disorder is available. The current lack of precision has generated a controversy concerning whether population screening is advisable. However, there is general agreement that testing for CF carriers should be offered to CF-affected families. The purpose of this study was to explore levels of understanding and feelings about carrier status and genetics of CF in affected families. Fifty-four adult CF siblings and their 30 spouses drawn from Children's Hospital, Oakland, and Children's Hospital, Boston, were interviewed, and transcripts were coded on various categories. The relationship between birth order and beliefs about carrier status was significant, with last-born siblings more likely to believe they are not carriers. Higher sibling resentment was found to be significantly related to willingness to abort an affected fetus, to more guilt, and to assumption of carrier status. Thirty percent of siblings believe that carrier status implies health difficulties. Increased feelings of guilt were significantly related to the belief that carrier status implies health problems and to the wish to be a carrier. Interestingly, beliefs regarding carrier status and the wish to be a carrier are not influenced by educational or income level. Some siblings have had their child tested for carrier status and others are planning to do so before the child reaches the age of 18 years. Perception of carrier status is strongly influenced by psychological factors.(ABSTRACT TRUNCATED AT 250 WORDS)