Abstract
Two control female Rhesus monkeys in long-term toxicity studies had multiple, palpable nodules in the breast regions. These subcutaneous nodules were 2–7 mm in diameter, firm, and freely movable. No progressive clinical change was observed during a follow-up of 17 or 20 months. Microscopically, one excised nodule consisted of proliferated acinar epithelial cells arranged in lobular patterns. The cells were well differentiated and presented no features of neoplasia. A diagnosis of focal nodular hyperplasia was compatible with the microscopie findings and the clinical follow-up of similar nodules.
