The significance of nipple discharge lies in the insignificance with which it is regarded. Cancer detection requires an accurate appraisal and interpretation of the importance of nipple discharge in diseases of the breast. This review constitutes a statistical survey of 114 patients with nipple discharge who were operated on in the Johns Hopkins Hospital during the 10-yr. period 1940-50. All patients were hospitalized, and the validity of the diagnoses was based upon pathologic evidence obtained in the operating room by the scalpel rather than by the random probing of an aspirating trocar. The difficulties of our experience at the breast clinic have cautioned us against the peril of putting too much faith in the thin ice of diagnoses based on clinical impressions alone. Thus, we have intentionally excluded from this study all patients who were classified as outpatients and who have never had their clinical diagnoses verified by microscopic examination.