This prospective study aimed to determine which is the most precise modality for the pre-operative measurement of primary breast cancers: clinical palpation; mammography; or ultrasound. Analysis of the difference between the measurement of the maximum tumour diameter by these three modalities and by the histological measurement was performed in 210 cases. Clinical palpation tended to overestimate tumour size and gave the largest standard deviation of the difference. Ultrasound and mammography both gave a similar standard deviation of the difference, with ultrasound tending to underestimate tumour size. For all modalities, the standard deviation and the 95% confidence intervals of the difference increased with increasing tumour size. There is little difference between the precision of ultrasound and mammography in measuring tumour size. The wide 95% confidence intervals for any method of pre-operative tumour measurement should be considered when planning patient management.