The evaluation of the response of primary breast cancer to systemic therapy is difficult. Evaluable primary lesions may be assessed both by physical and by mammographic examination. In this study, response to therapy was evaluated after 4 cycles of CMF or CMF plus tamoxifen in 49 patients with locally advanced breast cancer entering a prospective randomized trial. In 35 patients response was evaluated by both physical examination and mammography. In some cases there was disagreement between physical examination and mammography in quantifying the magnitude of response. In 8 of 35 (22.9%), the overall response was overestimated by physical examination versus mammography, while in 3 of 35 (8.6%) the reverse was true. Taking into consideration different criteria in attributing the overall response, i.e. selecting physical examination only, mammography only, or the most favorable or the least favorable response between the two methods of assessment, the objective remission rates were 65.7%, 54.3%, 71.4% and 45.7%, respectively. The data suggest that both physical examination and mammography should be used in evaluating the response of primary breast cancer to a systemic treatment. Should these two methods yield contrasting results, the data obtained with each method should be reported. The best observed response may be employed in determining the overall response.