Purpose: Oncogenesis has been associated with prenatal exposure to phe
nytoin, concomitant with or independent of the fetal hydantoin syndrom
e. The majority of reported cases have been embryonal tumors of neural
crest origin and have occurred in the first 3 years of life. Patients
and Methods: We report a boy who was exposed to phenytoin throughout
gestation and later developed T-lymphocyte lymphoblastic lymphoma, a p
reviously unreported malignancy associated with in utero phenytoin exp
osure. Previously reported cases of neoplasia occurring after such exp
osure are tabulated. Conclusion: The actual transplacental oncogenic p
otential of phenytoin and the epidemiology of this association are poo
rly understood. Phenytoin-induced alterations in lymphocyte-mediated i
mmunosurveillance or oxidative metabolic clearance may he etiologic. I
nquiry into prenatal phenytoin exposure should be done in any child wh
o develops cancer, especially those who develop a rare tumor or presen
t with a more common tumor at an unusually young age. Continued docume
ntation of such cases will advance the understanding of phenytoin-asso
ciated transplacental oncogenesis.