Our experience of breast ultrasound in the differentiation of benign and solid malignant breast masses indicates a specificity of 93% for carcinoma; 7% of cancers were regarded as benign. Eight of 89 proven fibroadenomas were considered possibly malignant. Typical, and overlapping, features are described, and the world literature is discussed. Whereas most previous workers have used a water-path technique, we employ direct contact B-scanning, which is quick, simple, and apparently at least as successful. The small group of solid masses where there is overlap between fibroadenoma and carcinoma needs further study; this might be helped by histological correlation with the ultrasound appearances of malignancy, the causes of which are still debated. The exclusion of a focal solid mass in a patient with a breast lump can reduce the biopsy rate.