When both parents are carriers of cystic fibrosis (CF), each child they have has a 25% chance of inheriting the serious illness. This means there's a 25% chance with each pregnancy that the child will have CF, a 50% chance that the child will be a carrier like the parents, and a 25% chance that the child will neither have CF nor be a carrier. It's important for couples who are carriers of CF to consult with a genetic counselor to understand the risks and options for family planning.