The aim of this study was to evaluate and to compare in terms of toxicity the modulations of dose intensity of cyclophosphamide and doxorubicin in adjuvant chemotherapy for high-risk breast cancer. Four cycles of sequential high-dose chemotherapy with doxorubicin and cyclophosphamide (AC), supported with G-CSF and peripheral blood stem cells (PBSC) were administered to 81 women. Three successive cohorts were studied: doxorubicin (75 mg/m(2)) + cyclophosphamide (3000 mg/m(2)) every 21 days (group 1), doxorubicin (75 mg/m(2)) + cyclophosphamide (3000 mg/m(2)) every 15 days (group 2), and doxorubicin (75 mg/m(2)) + cyclophosphamide (6000 mg/m(2)) every 21 days (group 3). Seventy-five patients received four cycles of treatment with a total of 310 cycles administered. The received dose intensity of doxorubicin was higher in group 2 and that of cyclophosphamide was lower in group 1 than in the other two groups. Hematological and extra-hematological toxicities, as well as the number and duration of hospitalizations for toxicity, were significantly higher in group 3. We conclude that the group 3 regimen is associated with toxicities comparable to autologous transplantation. Increasing dose intensity of doxorubicin and cyclophosphamide is feasible in an outpatient setting and safe in groups 1 and 2 with the support of hematopoietic factor and PBSC.