This study is an efficient Phase III trial designed to prevent skin cancers using a-difluoromethylornithine (DFMO).  DFMO is a chemopreventive agent that inhibits the promotion and early progression of many animal epithelial and skin cancers.  However, DFMO has not been subjected to a pivotal controlled Phase III trial.  Prior studies by University of Wisconsin investigators have shown that DFMO can be safely given to human subjects and that it inhibits skin ornithine decarboxylase (ODC), induced in vitro by the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA).  Our primary hypothesis is: Will subjects with a prior history of one or more skin cancers taking DFMO for average of four years have a decrease in the rate of new skin cancers as compared to placebo controls?  Our secondary hypotheses are: Will TPA-induced ODC inhibition be validated as an intermediary marker for DFMO as a chemoprevention agent?  Will inhibition of polyamine levels in the skin of subjects on DFMO be validated as an intermediary marker for DFMO as a chemoprevention agent?  Will DFMO administration be acceptable over the 3-5 years of treatment?