It has been suggested that interval breast cancer (breast cancer found within 12 months of a negative screening examination) exhibits a more virulent behavior than those found by the screening clinic. To test this hypothesis, 120 new cases of breast cancer were retrospectively reviewed from our Breast Screening Center. These patients were divided into three groups--interval cancer cases, lesions found at initial screening, and lesions found at subsequent screening. Statistically significant differences seen in our interval cancer cases included a higher percentage of positive axillary nodes, higher overall mortality rate, and lower 6-year survival rate as calculated by left table analysis. The remaining data, although not statistically significant, demonstrate a parallel trend toward a more malignant behavior. It is concluded that interval breast cancers are more aggressive forms of breast neoplasias.