PURPOSE To analyze histomorphometric consequences of the uterine arteries embolization (UAE) in the uterine tissue, especially by collagen tissue quantification through uterine biopsy, before and after treatment of uterine leiomyoma. METHODS 15 patients with symptomatic leyomioma and/or infertility, submitted to UAE, participated in the study according to the study exclusion criteria, after having signed an informed consent. Uterine biopsy was performed in the secretory phase of the menstrual cycle, before and three months after the procedure, to evaluate the collagen. After the histological processing of the material, 3 micro slices were prepared, some of them dyed with hematoxiline-eosin (HE) and others with the specific dye for collagen fibers (Picrosirius red). Then, the slides were examined and interpreted, and the collagen quantified. The amount was calculated as the percent of the area composed by collagen, and the result expressed in mean+/-standard deviation (SD). Data has then been submitted to statistical analysis by Student's paired t test (p<0.05). RESULTS The presence of smooth muscle cells was observed in the biopsies performed before the treatment, surrounded by a rich network of collagen fibers, which are part of the tumor, blood vessels and fibroblast nuclei. On the slides of biopsies performed after the treatment, it was observed the presence of widespread coagulation necrosis, vascular thrombosis, calcification and lymphoplasmocitary infiltration areas and clear reduction of the collagen component. The percentage of collagen fibers was higher in the pre-UAE group (84.07+/-1.41), than in the post-UAE (81.05+/-1.50) group, with p<0.0001, and 95% confidence interval (CI95%) from 2.080 to 3.827. CONCLUSION The quantitative and qualitative collagen reduction clearly shows that the proposed treatment is efficient in reducing the tumoral mass, composed mainly by collagen fibers intermingled with neoplasic smooth muscle cells. Nevertheless, complementary studies are needed to investigate the functional and biological consequences of these histological changes.