Juvenile papillomatosis (JP) is a breast tumor of young women featuring atypical papillary duct hyperplasia and numerous cysts, first described as a clinicopathologic entity in 1980. The disease is of interest because of the youth of these patients and the fact that the pathologic elements resemble those considered to be precancerous in older women. This report summarizes the findings in 180 patients enrolled in the Juvenile Papillomatosis Registry. The mean age at diagnosis was 23 years (range, 12-48 years; median, 21 years; mode, 18 years). No relationship was found with parity, age of menarche, or use of birth control pills. Fifty patients (28%) reported that one or more relatives had had breast cancer. Twelve (7%) patients reported a first degree relative (mother, 10; sister, 2), whereas in 38 (21%) breast cancer was limited to one or more secondary relatives, most often maternal (23/39 or 59%). Persistent (or recurrent) JP was found in 25 patients who had a subsequent biopsy, and 8 patients had bilateral JP. Pathologic analysis revealed focal epithelial necrosis, a feature often associated with carcinoma, in 30 (17%) cases predominantly in those with the most severe cytologic atypia of hyperplastic duct lesions (77%). Positive family history was reported by 33% (10/30) of women with epithelial necrosis and 27% (40/150) without this finding. Seven patients (4%) had breast carcinoma diagnosed concurrent with JP (ipsilateral, 5 cases; contralateral, 2 cases). A previously unreported finding is the diagnosis of carcinoma subsequent to JP in two cases (contralateral intraductal and ipsilateral microinvasive duct, respectively) with intervals of 8 and 9 years, respectively. The nine patients with carcinoma and JP tended to be slightly older at the time JP was diagnosed (mean, 27 years) and had a higher frequency of positive family history (56%) than those without breast cancer (mean age, 23 years; positive family history, 26%). These findings confirm earlier evidence that JP is a marker for families at risk and now indicate that the patient with JP may herself be in jeopardy of developing breast cancer coincidentally or at a later date. Wide local excision is adequate to control the lesion in most cases. Careful clinical surveillance is indicated for any woman who has juvenile papillomatosis and for her female relatives. The early age of onset of this unusually florid proliferative disease, and family history association, suggest there may be an underlying hormonal disturbance or genetic abnormality.