Organ transplant patients, an immunosuppressed population, are at high risk for developing squamous cell carcinoma (SCC) that arise from preneoplastic areas of epidermal dysplasia called actinic keratoses (AK). Early treatment of AK is critical to prevent progression to invasive SCC, but current treatments are largely ineffective, so that invasive and metastatic SCC occur frequently in these patients. Photodynamic therapy (PDT) is a modality that combines topical prodrugs, either aminolevulinic acid (ALA) or its methyl ester (mALA), which are selectively taken up by tumor cells and metabolically converted to protoporphyrin IX (PpIX). PpIX is a photosensitizer that absorbs visible light; illumination of preneoplastic cells containing high PpIX levels causes selective cell killing. PDT is an attractive modality because it treats a large surface area, is noninvasive, and is now considered superior to most other standard treatments for AK, including topical 5- fluorouracil cream (5FU). Yet in transplant patients, even PDT remains unsatisfactory, with recurrence rates for AK of 20-40% at 1 year post-therapy. The current proposal will test the hypothesis that PDT efficacy can be improved by combining PDT with a short (6-day) pretreatment regimen of topical 5-FU. Our preclinical animal studies showed that a short-contact 5-FU regimen can stimulate a 3-fold increase in PpIX levels in skin tumors in mice. We propose to test the hypothesis that 5-FU/PDT combination therapy will enhance PpIX levels in AK lesions in humans, and may prove more effective for treatment and prevention of AK, than PDT alone. This pilot study will enroll 40 subjects. One group of 20 subjects will be solid organ transplant recipients (status-post kidney or liver transplantation) with 4 or more AK lesions on the face or scalp. The second group will consist of 20 normal controls (non-transplant individuals with 4 or more AK), similarly treated, to control for effects of immunosuppression. Biochemical changes in PpIX levels (primary endpoint) and indicators of clinical effectiveness in terms of lesion resolution, lesion recurrence, and changes in biomarkers in skin and blood (secondary endpoints) will be examined in this exploratory study. Patients will be randomized to receive topical 5-FU pretreatment (daily for 6 days) on one side, and no pretreatment on the contralateral side. On day 7, levels of PpIX in lesions will be measured using fluorimetric monitoring: (i) before mALA application; (ii) at 3 hr after mALA application; and (iii) immediately after exposure to therapeutic light (37 J/cm2 of red light). Patients will be followed every 3 months for 1 year after PDT to determine AK clearance and AK incidence. In summary, Aim 1 will test whether 5-FU pretreatment can selectively increase photosensitizer (PpIX) produced within AK lesions. Aim 2 will test the 5-FU/PDT combination regimen in terms of clinical safety and efficacy, and usefulness of predictive biomarkers. This study will provide potential benefits to public health by establishing proof-of-principle for a therapeutic regimen that may substantially delay the onset of skin cancers in organ transplant recipients, a high-risk population for whom no effective option is currently available.