The aim of the study was to test the hypothesis, if there were subgroups of early breast cancer patients in which sole brachytherapy (BT) of the tumor bed was a feasible and safe treatment option after breast conserving surgery (BCS). Forty four prospectively selected patients with Stage I-II breast cancer were entered into a protocol of postoperative tumor bed irradiation using interstitial high dose rate (HDR) implants. The HDR fractionation schedules were calculated according to the linear quadratic model. In 8 patients 7 x 4.33 Gy, in the other 36 patients 7 x 5.2 Gy were delivered to the tumor bed with 2 cm margin. The treatment planning was based on the 3 dimensional (3D) reconstruction of the clipped excision cavity, catheters and skin points. A conformal semi-3D dose planning was used. The side effects were assessed by mammograms, MRI- and clinical examinations. At a median follow up of 20 (7-36) months 1 (2.3%) local and 1 (2.3%) regional failure was observed. Distant metastasis did not occur. The cosmetic results were judged to be excellent in each case. G2 radiation side effects were observed in 2 (4.5%) cases. Postoperative sole BT of the tumor bed with careful patient selection and adequate quality assurance seems to be a feasible alternative to whole breast radiotherapy after BCS. Sole BT shortens the time of radiotherapy from 5-6 weeks to 5 days, and reduces the costs of treatment. The skin and volume sparing effect of interstitial irradiation may decrease the side effects of radiotherapy. A randomized study is in progress to define which subgroups of patients should be candidates for BT alone after BCS.