We studied six pregnancies occurring in four patients with chronic myeloid leukaemia, diagnosed before pregnancy in one case and during the first pregnancy in three cases. For one patient the first pregnancy ended in spontaneous abortion but a second pregnancy, during which she was treated only by leucapheresis, was essentially uneventful although the baby was born with spina bifida and a small meningomyelocele and talipes. A second patient delivered a normal baby after treatment by leucapheresis alone during pregnancy. A third patient was treated with hydroxyurea from 16 weeks in her first pregnancy and with leucapheresis and hydroxyurea during the second pregnancy. Both babies were normal. A fourth patient was treated with busulphan in the third trimester of pregnancy and subsequently by leucapheresis immediately before delivery of a normal baby. Pregnancy may proceed normally in patients with chronic myeloid leukaemia; leucapheresis may be advisable in the first or second trimester but cytotoxic drugs can usually be administered in the later stages of pregnancy without undue risk to the fetus.