Following is the information from THE BREASTFEEDING ANSWER BOOK (1997)
on Cystic Fibrosis in mothers (there is a separate section on
breastfeeding the infant with CF.):

Alice Ziring, Mercer Island, WA

"Cystic fibrosis is a congenital disease that affects breathing and
digestion.  Until recent years those with cystic fibrosis rarely
survived into adulthood, but with improved treatment, women with cystic
fibrosis are having babies.

The individual with cystic fibrosis secretes unusually thick, gluey
mucus and the sweat is unusually salty.  The thick mucus clogs the
bronchial tubes in the lungs, causing breathing difficulties, and blocks
the digestive enzymes from leaving the pancreas, causing incomplete
digestion.  Cystic fibrosis can be milk to severe.  Some cases can be
detected only through laboratory tests, while some are extremely
serious.

With incomplete digestion and absorption of nutrients, maintenance of
proper weight is a major concern and special enzymes are sometimes taken
to aid in digestion.  (For information on breastfeeding the baby with
cystic fibrosis, see the chapter "The Baby with Special Needs.")

Although information is scarce, studies have found that mothers with
cystic fibrosis can breastfeed their babies and produce milk with normal
concentrations of protein, fats, and minerals (Michel and Mueller 1994;
Shiffman 1989).  Several case reports confirm that women with cystic
fibrosis have breastfed babies who have maintained normal growth (Michel
and Mueller 1994; Smith 1992).  Although one case report described a
mother whose milk had elevated sodium levels, this mother had not been
breastfeeding and had expressed her milk for study only.  Elevated
sodium levels are normal during breast involution and may have been
unrelated to her cystic fibrosis (Alpert and Cormier 1983).

Because cystic fibrosis impairs normal digestion, a mother should have
her nutritional needs carefully monitored before, during, and after
pregnancy, as well as during lactation.  If the mother is having
difficulty maintaining her weight or her health, or her baby is not
gaining well, breastfeeding may not be recommended.  Michel and Mueller
(1994) suggest that women with adequate weight and stable cystic
fibrosis be considered candidates for breastfeeding."