Because a "negative" mammogram that is followed by a biopsy diagnostic of cancer is a matter of deep concern, a retrospective review was conducted of 48 such missed diagnoses at four Breast Cancer Detection Centers. The study group comprised 40,000 women participating in breast cancer screening examinations. From 3,271 biopsies during screening, 499 cancers had been found. Biopsies in the interval between screening examinations totaled 630 and yielded 48 malignancies. These 48 interval cancers were studied in an attempt to discover why they were not found on the preceding mammographic examination. Three major categories of error were disclosed and each is discussed: (1) poor radiographic technique; (2) absence of radiographic criteria of cancer; (3a) obvious oversight by the radiologist; and (3b) lack of recognition of subtle radiographic signs. This last reason is discussed in detail in the belief that better recognition of these indirect radiographic signs will lead to more accurate diagnoses, particularly in early cancers.