Fine needle aspiration (FNA) biopsies of 1,598 breast masses were performed between 1983 and 1989, and of them, 48 were from women aged 30 and under for whom a cytologic diagnosis was made by FNA and histologic follow-up was available. In 37 (77%) of the cases, both the cytologic and histologic diagnoses were benign. Fibroadenoma (20/37) and fibrocystic changes (14/37) were the most common benign lesions aspirated. Eight (17%) FNAs showed cytologic atypia. Four of these atypical lesions proved to be benign (two fibroadenomas, two fibrocystic changes). Epithelial proliferation in fibroadenomas and fibrocystic changes and cellular stroma in a fibroadenoma mimicking phylloides tumor were the causes of atypia in these biopsies. Four of the eight atypical lesions were shown to be carcinoma at biopsy (three infiltrating duct, one atypical medullary). Low cellularity, epithelial cohesiveness mimicking a fibroadenoma and background lactational changes in a pregnant patient were the causes of the atypical, rather than unequivocally malignant, diagnoses in these cases. In three patients (6%), a diagnosis of carcinoma was made by FNA and confirmed histologically (all were infiltrating duct carcinoma). Although most breast masses in women aged 30 and under are benign, cytologic atypia in a breast fine needle aspirate in this age group warrants a surgical biopsy. Clinical follow-up alone may be appropriate for young women with clinically nonsuspicious breast masses without cytologic atypia.