Xeroderma pigmentosum (XP) is an autosomal recessive disease with tumor formation on sun-exposed areas of the skin and eyes. Cells from most XP patients are deficient in repairing DNA damaged by ultraviolet (UV) light as shown by a reduced rate of tritiated thymidine (3HTdR) incorporation during their DNA repair synthesis. We have studied such repair synthesis in conjunctival cells from an XP patient with a conjunctival epithelioma and from normal cadaver conjunctiva. Cultured conjunctival cells were irradiated with UV light and then incubated with 3HTdR. Autoradiograms were prepared and showed that UV radiation induced a considerably slower rate of DNA repair synthesis in the XP cells than in normal cells. Many of the ocular abnormalities of XP, including tumor formation, may be the result of this defective DNA repair process.