Nucleolin is over-expressed in malignant tumors and is used as a marker for cell proliferation and to reliably predict tumor growth rate.  However, it is not known whether nucleolin expression is directly involved in or is a consequence of carcinogenesis. Using GST-pull down assays, we have determined that the recombinant nucleolin interacts with the Proliferating Cell Nuclear Antigen (PCNA). Co-immunoprecipitation assays indicate that the nucleolin-PCNA interaction also occurs in intact cells and this interaction increases after exposure of colon carcinoma RKO cells to UV radiation. Moreover, our data indicate that PCNA and nucleolin co-localize in some areas within the RKO cell nuclei.  The functional significance of this interaction is evaluated on Nucleotide Excision Repair (NER) since PCNA is a primary mediator of this cellular function. Our data indicate that overexpression of nucleolin decreases the repair efficiency of UV damaged plasmid DNA in RKO cells. Co-transfection with PCNA can rescue this effect in vivo. Furthermore, reduction of nucleolin protein levels increases DNA repair efficiency in RKO and CHO cells and consequently increases cell survival. These data indicate that the direct interaction of nucleolin with PCNA inhibits NER efficiency of UV damaged DNA. This effect could contribute to carcinogenesis and aging in cells over-expressing nucleolin.