This clinical trial will test a new combination of 5-fluorouracil cream (5FU) and methylaminolevate photodynamic therapy (MAL-PDT), versus MAL-PDT alone, as treatment for actinic keratoses (AKs) in immunosuppressed organ transplant recipients (OTRs) and an immunocompetent control group. Objectives: 1) Determine whether topical pretreatment with 5-FU selectively increases the amount of photosensitizer (PpIX) produced within AK lesions, relative to non-pretreated lesions. 2) Determine whether the combination treatment improves lesion resolution and reduces the incidence of new AKs. 3) Determine whether biomarkers in tissue and blood are predictive of patient responsiveness to 5FU (PpIX induction, new lesion incidence, and clinical toxicity).