Since chemotherapy for metastatic breast cancer is not curative, consideration of the quality of life is important in selecting a treatment regimen. We conducted a randomized trial comparing continuous chemotherapy, administered until disease progression was evident, with intermittent therapy, whereby treatment was stopped after three cycles and then repeated for three more cycles only when there was evidence of disease progression. Each approach was tested with doxorubicin combined with cyclophosphamide or with cyclophosphamide combined with methotrexate, fluorouracil, and prednisone. Intermittent therapy resulted in a significantly worse response (P = 0.02 by Mann-Whitney test), a significantly shorter time to disease progression (relative risk based on proportional-hazards model, 1.8; 95 percent confidence interval, 1.4 to 2.4), and a trend toward shorter survival (relative risk, 1.3; confidence interval, 0.99 to 1.6). The quality of life was expressed as linear-analogue self-assessment scores for physical well-being, mood, pain, and appetite and as a quality-of-life index. It improved significantly during the first three cycles, when all patients received treatment. Thereafter, intermittent therapy was associated with worse scores for physical well-being (by 23 percent of scale; 95 percent confidence interval, 11 to 35 percent), mood (25 percent; 13 to 37 percent), and appetite (12 percent; 0 to 24 percent) and for the quality-of-life index as indicated by the patient (14 percent; 5 to 23 percent) and the physician (16 percent; 7 to 26 percent). Changes in the quality of life were independent prognostic factors in proportional-hazards models of subsequent survival. We conclude that, as tested, continuous chemotherapy is better than intermittent chemotherapy for advanced breast cancer.