Background: Breast cancers are becoming more frequently diagnosed at early stages with improved longterm outcomes. Late normal tissue complications induced by radiotherapy must be avoided with new breastradiotherapy techniques being developed. The aim of the study was to compare dosimetric parameters of planningtarget volume (PTV) and organs at risk between conformal (CRT) and intensity-modulated radiation therapy(IMRT) after breast-conserving surgery. Materials and Methods: A total of 20 patients with early stage leftbreast cancer received adjuvant radiotherapy after conservative surgery, 10 by 3D-CRT and 10 by IMRT, witha dose of 50 Gy in 25 sessions. Plans were compared according to dose-volume histogram analyses in terms ofPTV homogeneity and conformity indices as well as organs at risk dose and volume parameters. Results: The HIand CI of PTV showed no difference between 3D-CRT and IMRT, V95 gave 9.8% coverage for 3D-CRT versus99% for IMRT, V107 volumes were recorded 11% and 1.3%, respectively. Tangential beam IMRT increasedvolume of ipsilateral lung V5 average of 90%, ipsilateral V20 lung volume was 13%, 19% with IMRT and3D-CRT respectively. Patients treated with IMRT, heart volume encompassed by 60% isodose (30 Gy) reducedby average 42% (4% versus 7% with 3D-CRT), mean heart dose by average 35% (495cGy versus 1400 cGywith 3D-CRT). In IMRT minimal heart dose average is 356 cGy versus 90cGy in 3D-CRT. Conclusions: IMRTreduces irradiated volumes of heart and ipsilateral lung in high-dose areas but increases irradiated volumes inlow-dose areas in breast cancer patients treated on the left side.