With the aim of increasing the dose intensity of chemotherapy in breast cancer, 14 patients with stage II-IV breast cancer were treated with FEC chemotherapy at 2 week intervals together with granulocyte colony-stimulating factor (G-CSF) 5 micrograms kg-1 s.c. on days 2-14. Five of six patients completed six courses of 5-fluorouracil 600 mg m-2, epirubicin 60 mg m-2 and cylcophosphamide 600 mg m-2 within 11 weeks. Eight patients were treated with 5-fluorouracil 700 mg m-2, epirubicin 70 mg m-2 and cyclophosphamide 700 mg m-2 and four had dose-limiting toxicity with sepsis, thrombocytopenia or mucositis. All patients who received G-CSF had satisfactory neutrophil counts by day 15 of each course. Cumulative anaemia and thrombocytopenia were observed, but treatment at the first dose was tolerable. Seven of eight patients with measurable disease had partial responses. This regimen permits a 50% increase in dose intensity compared with conventional treatment at 3 week intervals and warrants further evaluation.