BACKGROUND: Patients’ compliance is a leading factor in achieving determined endpoints of a treatment process. Low patient compliance with prescribed therapeutic plan is very common in both ambulatory practice and clinical trials. Non-adherence of subjects in clinical studies can seriously jeopardize validity of results obtained during randomized clinical trials (RCTs).METHODS: Both systematic literature review and cross-sectional survey were conducted among physicians and patients to determine evaluation of therapy adherence by both groups. RESULTS: According to the analytical reviews in published articles, the most common definition of therapy adherence or compliance is “the ability and willingness of patients to abide to a prescribed therapeutic regimen”. There is a strong correlation between patient adherence to the therapeutic regimen determined by the study protocol of RCTs, and the validity of efficacy and safety evaluation of the investigated medicinal product. Analysis of the results from the conducted survey lead to the conclusion that 35% of physician’s state that the results of ongoing therapy are highly dependent on patient compliance. As far as patients are concerned, almost half of the respondents have answered they comply with the prescribed therapy and would evaluate the treatment process as successful. Both patients and medical professionals declare their willingness to participate in clinical trials and strictly stick to the requirements of the study protocol.Conclusion: These results decisively confirmed that the control of a disease is a bilateral process between the patient and the doctor. Patient satisfaction with their current therapy directly correlates with the results of treatment. Majority of patients evaluate their possible participation in a clinical trial as an opportunity to get an access to a better medical service. Both medical professionals and patients declare their strong motivation to comply with the therapeutic plan if it is a part of a clinical study protocol.