In the criteria used for assessment of response to treatment for advanced breast cancer the definition of no change (NC) is clear; however, there is no indication of the duration of stabilization required for patients to qualify for this category of response. We have made the assumption that NC is a worthwhile category of response if the overall time to progression (TTP) and survival of this group is not significantly different from patients with partial remissions (PR). Two hundred and sixty-three evaluable patients treated with endocrine therapy and 302 evaluable chemotherapy-treated patients were studied and the TTP and survival curves for PR and periods of NC from 1 to 6 months compared. For the endocrine-treated patients the TTP and survival curves for NC became non-significantly different from the PR curves after 4 and 5 months respectively. For chemotherapy-treated patients the TTP curves became non-significantly different from PR at 4 months and for survival the period was 3 months. In order to define NC as a useful category of response and to eliminate the possibility that NC taken for a shorter period could simply represent a slowly progressive tumour, we suggest that the minimum period of disease stabilization be taken as 5 months for both endocrine- and chemotherapy-treated patients.