Purpose: A double-blind randomized Phase II chemoprevention trial of α-difluoromethylornithine (DFMO) was conducted in a group of women at high risk for development of breast cancer. DFMO is an irreversible inhibitor of ornithine decarboxylase, the limiting enzyme of polyamine synthesis that is often up-regulated in breast cancer. Experimental Design: Study entrants were required to have random periareolar fine-needle aspiration cytology prior to entry that exhibited hyperplasia or hyperplasia with atypia, as well as a mammogram and clinical breast exam judged as not suspicious for breast cancer and no clinical hearing loss. Subjects were randomized to 6 months of oral DFMO (0.5 g/m2/day) or placebo, followed by repeat fine-needle aspiration and biomarker assessment. The main study end point was an improvement in cytologic pattern. Results: Of 119 subjects entered, 96% completed the study and were evaluable for the main study end point. A modest reduction (28%) in average total urine polyamines was obtained in the DFMO group, but there was no reduc tion in the spermidine:spermine ratio. There was no difference in cytologic improvement between DFMO and placebo. Likewise, there was no difference between DFMO and placebo for the secondary end points of breast molecular marker changes (immunocytochemical expression of proliferating cell nuclear antigen, p53, and epidermal growth factor receptor), mammographic breast density, serum insulin-like growth factor I: insulin-like growth factor binding protein 3 ratio, adverse events, quality of life indices, or subsequent cancer development. Conclusions: DFMO at a dose level of 0.5 g/m2/day administered for 6 months does not modulate breast risk biomarkers tested in this study.