Cyclophosphamide (CTX) is an active drug in breast cancer and presents a well-established dose-response relationship. To explore further this relationship, the present pilot study investigated the therapeutic efficacy of cyclophosphamide at intermediate dose in two groups of untreated patients with advanced breast cancer. Nine women received the drug alone at 3-4 g/m2 i.v. every 2 weeks for a total of three doses. The same dose schedule was also given to 11 women following the administration of four cycles of Adriamycin, at 75 mg/m2 i.v. every 3 weeks. We documented one partial remission in untreated women and four partial responses in Adriamycin-treated patients. The major toxicity was represented by leukopenia and neutropenia. Myelosuppression was relevant but of short duration, and the use of G-CSF appeared useful in controlling this side effect. In spite of the high dose intensity of the present cyclophosphamide dose schedule (9 g/m2 in 4 weeks), i.e., almost three times superior to that conventionally employed, present results do not suggest its superiority over the current chemotherapeutic regimens utilized in advanced disease.