Photodynamic therapy (PDT) using aminolevulinic acid or its methyl ester, methyl-aminolevulinate (MAL), is an increasingly recognized approach for treating squamous neoplasia of the skin. Advantages of MAL-PDT include its ability to cover broad diseased areas (field treatment), and to do multiple sessions with little-to-no risk of scarring or mutagenesis. MAL-PDT is especially valuable in certain populations at high risk for skin cancer, including Caucasian patients with extensive solar damage, and organ transplant recipients (OTR) who take immunosuppressive drugs to prevent graft rejection. The latter group has a 65-200 fold increased risk of developing squamous cell carcinoma (SCC), a major cause of mortality. Therapeutic options for those patients, other than frequent surgeries, are very limited. Topical 5-Fluorouracil (5-FU), frequently prescribed in normal patients for pre-SCC of the skin, is only minimally effective in the OTR group. MAL-PDT, however, has ~40% efficacy for pre-SCC in OTR patients. Based upon our preclinical studies in mouse tumor models, which showed that preconditioning with 5-FU can drive higher accumulation of target protoporphyins (PpIX), we proposed a rational combination regimen of 5-FU and MAL-PDT in humans. A clinical trial was designed to test the hypothesis that a combination of 5-FU followed by MAL-PDT will elevate PpIX levels and achieve better clinical outcomes in high-risk OTR patients. Primary endpoints include PpIX levels and biochemical markers (p53) measured noninvasively and in skin biopsies. Lesion clearance and recurrence (via photographs and clinical exam) are secondary endpoints. Ongoing results of this clinical trial are presented.