This randomized, intra-patient comparative study is designed to investigate the combination regimen of 5-FU and PDT, versus PDT alone, for its ability to generate significantly elevated levels of the target photosensitizer, PpIX, in lesions of actinic keratoses (AKs) and to more effectively treat and prevent recurrence of AKs. The target population comprises patients with solid organ transplants (renal, hepatic), as well as normal (immunocompetent) subjects to control for possible influences of immunosuppression.