Postmarketing data and data from the U.S. National Transplantation Pregnancy Registry have prompted a black box warning for mycophenolate mofetil (CellCept) including a change of pregnancy category. CellCept, a product used to prevent the rejection of a transplanted heart, kidney, or liver, has been associated with heightened risks of miscarriage in the first trimester and of congenital malformations comparable to those observed in studies of the drug used in animals (external ear and facial abnormalities, such as cleft lip and palate, and anomalies of the heart, esophagus, distal limbs, and kidney). The drug has been put into Pregnancy Category D, indicating evidence of risk to the fetus. Nurses should instruct patients of childbearing age to use effective contraception while taking CellCept.