Breast cancer is the most frequently diagnosed cancer in women, and 70% of diagnoses are the hormone-dependent type. The high survival rate is a result of effective endocrine treatment that includes aromatase inhibitors (anastrozole, letrozole and exemestane) and tamoxifen. The use of AET shows numerous benefits, but patient adherence is not satisfactory. Long-term use of therapy (from 5 to 10 years) also causes numerous side effects that influence the quality of life, so these are considered factors of poor adherence.The aim of this thesis was to examine the type and frequency of side effects caused by AET depending on duration of therapy with aim of improving treatment access and adherence. The research was conducted at Clinical Hospital Center (KBC) Zagreb from September to November 2021. Patients were of adult age, average of 57 years, with hormone-dependent breast cancer stage 0-III who are treated with anti-estrogen therapy for more than 3 months.First part of the questionnaire collected sociodemographic and clinical data, while the second part of the Functional Assessment of Cancer Therapy-Endocrine Symptoms (FACT-ES) questionnaire was used to examine the frequency and type of endocrine side effects.The study showed that patients on tamoxifen therapy have a higher prevalence of vascular side effects. The prevalence of the side effect hot flushes was the highest among all subjects regardless of the length of the therapy (more than 70%).In patients on AI therapy, the frequency of the side effect hot flushes decreases with the duration of therapy, while the side effect pain in the joints is the most frequent, regardless of the length of therapy (80%).Research of this type was conducted for the first time on patients with hormone-dependent breast cancer in Croatia and contributes to a better understanding of factors that can improve adherence. Positive attitude of patients towards therapy is influenced by their relationship with the healthcare professional, therefore it is necessary to build trust that will result with mutual information about the outcomes of the therapy, as well as its expected side effects.