The goal of this study was to compare findings on initial and subsequent screening mammograms to determine the prognostic significance of screening-detected abnormalities.All 3386 abnormal examinations from a 9-year mammographic screening program were studied. An initial examination was defined as one for which there were no prior films available for comparison (even if one or more prior examinations had been performed); the remainder were called subsequent examinations. The principal mammographic feature of each abnormality was recorded, as well as whether a biopsy was performed. For all screening-detected cancers, we also determined several surrogate markers of prognosis (tumor size, presence of axillary lymph node metastasis, and tumor stage). These various parameters were analyzed as a function of initial versus subsequent screening.The frequency of abnormal examinations was more than 2 times greater for initial examinations (7%) than for subsequent examinations (3%). Only minor differences were noted...