In a prospective study, use of serial ultrasound (US) for monitoring tumour response to pro-adjuvant chemotherapy was assessed in 16 patients. Comparison was made with mammographic and pathological tumour size measurements. Clinical and radiological response to treatment was assessed using UICC (International Union Against Cancer) criteria. Comparison of clinical and US response to treatment showed some agreement in 60% and disagreement in 40%. This was comparable with clinical versus mammographic responses (55% and 45%). Correlation between calliper and pathological measurement was similar to that between US and pathological measurement (r = 0.51, P = 0.05; r = 0.50, P < 0.05). Mammography showed poorer correlation (NS). For assessment of final tumour size, US clinical measurements were comparable and better than mammography. US may be a useful tool in monitoring the response of breast tumours to pro-adjuvant therapy.