127 women attending the Edinburgh Breast Screening Centre were found to have small clusters of microcalcifications (maximum diameter 1 cm) of a potentially malignant nature. Using criteria previously postulated,1 they were classified into three groups: uniform localised, non-uniform widespread, and non-uniform localised, using hand-held magnification. In each group, the risk of malignancy was assessed in the short and long-term. The cancer detection rate of the uniform localised calcification, 67 women, 16 (24%) biopsied, was 0% immediately and 1.5% when a further biopsy was carried out after 2 years. For non-uniform widespread calcification, the cancer detection rate, 49 women, 31 (63%) biopsied, was 6 (12%) immediately and 8 (16%) when a further 3 biopsies were carried out up to 2 years later. The cancer detection rate of non-uniform localised calcifications, 11 women, all biopsied, was 7 (64%). The use of this classification of microcalcification provided a significant predictor for identification of breast cancer (p<0.001). Conventional risk factors were also examined, but none markedly improved discrimination with only family history approaching statistical significance (p = 0.09). The utility of the method in the screening situation is discussed.