Despite its importance for clinical policy, there is little research on the age at which ultrasound or mammography is the preferred initial diagnostic test in women with breast symptoms. We analysed data from a series of 3799 consecutively presenting cancers. The overall sensitivity was 77.6% for mammography and 80.5% for ultrasound. Sensitivity increased with age in both mammography and ultrasound, though more steeply for mammography. In the 975 women who had both tests, ultrasound had a higher sensitivity than mammography in women younger than 62 years of age, whereas mammography had a higher sensitivity than ultrasound in women older than 62 years. However, if the test results of the 2393 women who had mammography only are compared with the ultrasound results of the 975 women who had both tests, the 'cross-over' age at which the sensitivity of the two tests is equal occurs at the earlier age of 48 years. The presence of a palpable finding increases the sensitivity of ultrasound but does not influence the sensitivity of mammography.