In hormone type of early breast cancer, high survival is the result of very effective endocrine treatment options. However, long-term adjuvant endocrine therapy (AET) which includes aromatase inhibitors (anastrozole, letrozole, and exemestane) and tamoxifen leads to a number of side effects resulting from the antiestrogenic effects of these drugs. In premenopausal women, the side effects of AET may be even more pronounced due to a sudden and strong estrogen suppression which is the consequence of intensive endocrine therapy. According to the literature, the side effects of AET affect the quality of life and adherence of patients. Since AET is a chronic therapy taken through the period from 5 to 10 years, it is very important to identify women at high risk for certain side effects and the women who are expected to have more severe forms of certain side effects. Many of the most common side effects can be alleviated by nonpharmacological and pharmacological approaches with an aim of maintaining high adherence and persistence as they are necessary for reducing the risk of disease recurrence and high survival rates of this group of patients. The aim of this diploma thesis was to investigate the type, frequency and severity of side effects that occur due to AET in women suffering from early breast cancer. Furthermore, this study also investigated the impact of gonadotropinreleasing hormone (GnRH) comedication on the type, prevalence, and severity of side effects that occur in AET. Participants in this study were women who suffered from stage I-III hormone-dependent breast cancer and who started AET at least three months before completing the questionnaire. This research included 329 women with an average age of 57 years. Subjects on tamoxifen had a statistically significant higher prevalence of vascular (hot flashes, cold sweat and night sweats) and vaginal (vaginal discharge, vaginal irritation) side effects compared to subjects on AI, while subjects on AI had a statistically higher prevalence of stronger bone pain. Also, the comedication with GnRH and AET significantly contributed to the prevalence and severity of side effects compared to treatment with endocrine drugs alone. Furthermore, the prevalence and severity of all the most common side effects, except bone pain, was higher in women treated with the GnRH + AET combination. This type of research was conducted for the first time on the population of patients with hormone positive breast cancer in Croatia and can contribute to a better understanding of adherence factors. The identification of the subpopulation of women with the highest risk of severe AET side effects is important as to target specific interventions and intensify pharmaceutical care for women whose quality of life is likely to be significantly impaired during AET and consequently at risk of nonadherence and failure in chronic treatment.