Background: Diabetes Mellitus (DM) is a global epidemic. Globally the number of diabetic complications is increasing. Therefore, it is important to find the reasons for the escalating number of diabetic complications and associated mortality. The main objective of this study was to evaluate patients’ knowledge and level of adherence to treatments, the cost associated with diabetic medications and the level of glycemic control.Methods: This study was a pretested, structured questionnaire-based cross-sectional survey with retrospective data collection from participants’ clinic books. The questionnaire was developed to obtain data on socio-demographic variables; patients’ knowledge on disease and medicines; patients’ dietary control and adherence to the medications and non-medicinal recommendations.Results: Patients (n = 207) attending the diabetes clinics of two teaching- and two base-hospitals in Sri Lanka were taken as the sample. The majority of the participants demonstrated moderate knowledge (63%) on DM. Of the participants, 46% had moderate adherence to pharmacological and nonpharmacological recommendations of healthcare providers and 55% showed good glycemic control. The monthly average drug cost for a DM patient was 270.10 Sri Lankan Rupees (LKR). There was a significant association between participants’ knowledge and level of adherence. However, there were no significant associations between participants’ knowledge and glycemic control, or patients’ adherence and glycemic control. There was a significant knowledge gap between the patients attending Teaching- and Base-hospitals.Conclusion: The majority of the diabetic patients had either moderate or above knowledge and adherence, but their glycemic control was sub-optimal and associated with factors other than patients’ knowledge and adherence.