Objectives: Determining the adherence to drug therapy for chronic patients with diabetes and determining possible differences in adherence between patients with diabetes type I, who are treated with parenteral therapy, and those with diabetes type II, who are treated with oral antidiabetic medications, and possible differences in adherence measured by two different scales (the Čulig scale and the Morisky-Green scale). The final objective is giving recommendations to patients and pharmacists in order to increase the degree of adherence. Based on those results, pharmacists will come to know the problem of possible non-adherence of patients and will be able to directly talk to the patients with recommendations while issuing medications to patients; especially regarding the medication dosage, regular intake, registering side effects or adverse effects, and all other factors related to prescribed therapy. Patients and Methods: The study is non-interventional, the protection of personal data is provided, and bioethical standards, i. e. four fundamental bioethical principles, are respected. The study was approved at the session of the Ethics Committee of Andrija Stampar Teaching Institute of Public Health on March 16 2015, and at the session of the Committee for Ethics of Experimental Research of the Faculty of Pharmacy and Biochemistry on February 11 2016. Patients with diabetes were randomly surveyed in 11 pharmacies: in the Zagreb area (Martićeva, Vrbik, Gundulićeva, and Rudeš) and in Ozalj, Gvozd, Rugvica, Zadar, Slavonski Brod, Gornja Vrba, and Donji Miholjac. The patients who are treated with oral antidiabetic medications were compared to those who are treated with parenteral insulin therapy. Two scales were used for the measurement of adherence to medication therapy: the Čulig scale and the Morisky-Green scale. The data were processed using appropriate statistical methods. The set hypotheses were tested using the statistical methods in order to determine statistically significant differences. Results: The study was carried out on 448 patients in order to determine their adherence to antidiabetic medication therapy. The number of respondents included in both scales was almost identical. That is, 223 respondents were interviewed with the Morisky-Green scale, and 225 respondents with the Čulig scale. However, there is a difference in number of respondents considering the way medication was administered, thus 347 persons used oral medications, while 101 persons used parenteral insulin therapy. The total non-adherence is relatively high (40.8%) and is somewhat higher with oral antidiabetics (42.4%) than with parenteral therapy (35.6%). The Čulig scale, which has a higher reliability coefficient (Cronbach alpha), shows lower degree of adherence with both oral (40.2% to 44.5%) and parenteral therapy (31.4% to 40.0%). Conclusion: The main problem with long-term therapy is significant reduction in adherence to medication. The main reason for insuficient clinical benefit of therapy is poor adherence. It causes medical and psychosocial complications of illness, decreases the quality of patients' life, and wastes healthcare resources. Patients need to be informed of the importance of regularly taking prescribed medication therapy. Also, in agreement with the patients, one should find the right way to remind them of taking the prescribed therapy.