We performed a pilot study to evaluate the safety, tolerability and clinical usefulness of accelerated hyperfractionated radiotherapy in patients treated with breast conserving therapy (BCT). The radiotherapy was consisted of 1.5 Gy per fraction twice daily to a total dose of 51 Gy to the whole breast with or without 9 Gy boost to the tumor bed in case of positive surgical margin. Thirty-seven patients were treated with accelerated hyperfractionated radiotherapy and 178 patients with conventional radiotherapy between September 1993 and December 1997. Acute or late skin toxicity of Grade 3 or more was not observed. One case of radiation pneumonitis requiring steroid therapy occurred in the conventional group. 8-year local-relapse-free survivals were 100% in accelerated group and 97.0% in conventional group (not significant: p = 0.59). The scores in quality of life at one year after the completion of radiation therapy were not different between the conventional and the accelerated group; symptomatic scores, global QOL scores, and scores by self-estimation for therapy. Admission charges in conventional and in accelerated group were not different, either. These data suggested that accelerated hyperfractionated radiotherapy in BCT was feasible and phase III study to compare conventional with the accelerated hyperfractionated radiotherapy should be planned to evaluate its clinical usefulness in BCT.