In this study, the ability of granulocyte colony-stimulating factor (G-CSF) to treat or prevent chemotherapy-induced oral mucositis in patients with advanced breast cancer was evaluated. A total of 14 patients who received intraarterial (i.a.) adriamycin (ADM) preoperatively were divided into two groups according to whether or not G-CSF was given. Thus, group A (n = 7) was given G-CSF and group B (n = 7) was not. G-CSF therapy reduced both the incidence and duration of ADM-induced oral mucositis, and a positive correlation was also seen between the incidence of mucositis and ADM-induced leukopenia (< 2,000/mm3). Group A was further divided into two subgroups according to whether G-CSF was given after or before the leukopenia had dropped below 2,000/mm3: group A-1 (n = 3) and group A-2 (n = 4), respectively. ADM-induced mucositis was observed in two of the three patients in group A-1, but in none of the four patients in group A-2. These results strongly support the idea that G-CSF can effectively treat and prevent ADM-induced oral mucositis.