A double-blind study was undertaken in man to determine if chloroquine could inhibit cutaneous carcinogenesis. Via a random sampling technique over 200 tumor prone individuals were placed on chloroquine, 250 mg. per day, or an identical appearing placebo. After 11 months, the patients taking chloroquine, when compared to the placebo group, developed fewer new tumors. This was demonstrated in the total number of new tumors and also in the percentage of individuals developing one or more new tumors. Statistically significant figures were found in the 6-17 month time period; however, the results were not significant at the .05 level at any other time period. The findings from this study suggest that chloroquine may have a moderate suppressive effect upon carcinogenesis.