Samples of uterine leiomyomatis and normal tissues taken from pa- tients after surgery were investigated using the Positron Annihilation Life- time Spectroscopy (PALS). Significant differences in all PALS parameters between normal and diseased tissues were observed. For all studied pa- tients, it was found that the values of the free annihilation and ortho- positronium lifetime are larger for the tumorous tissues than for the healthy ones. For most of the patients, the intensity of the free annihilation and ortho-positronium annihilation was smaller for the tumorous than for the healthy tissues. For the first time, in this kind of studies, the 3γ fraction of positron annihilation was determined to describe changes in the tissue porosity during morphologic alteration.