Breast conserving surgery followed by adjuvant radiotherapy and eventually by systemic treatment represent the current trend in therapy of the early-stage breast carcinoma. Local control and the final cosmetic effect are important factors in breast conserving therapeutic approaches. We evaluated 215 patients who underwent breast conserving surgery (BCS) followed by adjuvant radiotherapy (RT) in our institute between October 1996 and February 2004. External beam radiotherapy (EBRT) was performed using a Cobalt- 60 or linear accelerator (LINAC), the boost was administered via high dose rate interstitial brachytherapy (HDR BRT) employing the Gammamed afterloading system. Patient survival was evaluated using the Kaplan-Meier method (disease-free survival DFS, overall survival OS). Late radiotherapy effects were evaluated using the LENT scales. The cosmetic effect (CE) was rated on a 4-grade scale by the patient and a committee; the Breast Retraction Assessment (BRA) was used to objectively assess the extent of the breast deformation and areolar deviation. The median follow-up in our group of patients was 70 months (from 20 to 136 months). Local control of the disease after 5 years was achieved in 98.5% of the patients, DFS was 88.7%, the distant disease-free survival (DDFS) was 89.9% and the overall 5-year survival was 91,8%. Medium vs. heavy fibrosis were recorded in 31.2% vs. 4.2% of the patients, medium vs. heavy teleangiectasia in 11.2% vs. 14.0% of the patients, and medium vs. heavy pigmentations in 6.5% vs. 3.3% of the patients, respectively. In all other cases none or minimal late radiotherapy effects occurred. The total CE was significantly influenced by the extent of the surgery (smaller deformations following tumorectomy < 65 cm3), by the type and orientation of the surgical incision (better results with discontinuous scars then with radial continuous scars), by the depth of the applied HDR BRT needles, by the rate of intermediate and severe postradiation late effects, plus by the value of the objective BRA parameters. Our data show that the HDR interstitial brachytherapy boost offers both excellent local control and favorable cosmetic effect to the patient, as long as the indications are followed closely. This therapeutic approach is suitable for treatment of tumors localized deeper than 2.8 cm under the surface and in patients with voluminous breasts.