Burkitt lymphoma was diagnosed in a patient during the third trimester of pregnancy. Treatment with a low-dose, seven-day course of intravenous cyclophosphamide gave a good response which permitted the pregnancy to be carried to term, with delivery of a normal infant. When the mother received cyclophosphamide during lactation while the baby was breast-fed, the infant's leukocyte and platelet counts were rapidly depressed. Results in this patient and a survey of reported cases in which cyclophosphamide was administered during pregnancy and lactation lead to the conclusions that (1) low-dose intravenous cyclophosphamide therapy is not hazardous to the fetus during late pregnancy; (2) mammary concentration of the drug is too toxic to the infant's bone marrow; and (3) breast-feeding the baby should be suspended during the period the mother is receiving cyclophosphamide treatment.