Background: Poor adherence to treatment regimen especially in chronic illness has been a significant contributor to poor treatment outcomes, increased morbidity and mortality. Non-adherence to treatment is a multifaceted problem that has continued from ancient times till now. Recognition of this problem has spurred a lot of research efforts worldwide to identify risk factors especially modifiable risk factors for non adherent behaviour. This study is designed to make some contributions along this line. Objectives: (a) To assess the point prevalence of treatment adherence in Schizophrenia and compare it with Bipolar disorder and Diabetes mellitus. (b) To compare the socio-demographic and clinical variables amongst the study populations. (c) To assess the impact of socio-demographic and clinical variables on treatment adherence. (d) To determine reasons for treatment non-adherence in the study populations. Method: A total of 369 subjects (consisting of 123 each of subjects with Schizophrenia, Bipolar disorder and Diabetes mellitus) were recruited for this study. A socio-demographic questionnaire was used to assess socio-demographic characteristic of the subjects. Morisky adherence questionnaire, attitude to treatment questionnaire, medication prescription knowledge questionnaire, resistance to treatment questionnaire and QAM-Q questionnaire were used to generate data on clinical and socio-demographic variables. Results: Treatment adherence was significantly higher in subjects with Diabetes mellitus (70%) than in Schizophrenia (52%) or Bipolar disorder (37.4%) (p=<0.01). Adherence to treatment was significantly affected in all study groups by supervision of drug intake (x2=36.65, df=2, p=<0.01), number of tablets taken in a day (x2=6.21, df=2, p=<0.01) and attitude to treatment (x2=32.55, df=2, p=<0.01). The socio-demographic and clinical variables that made contributions to treatment adherence showed significant differences amongst subjects with Schizophrenia, Bipolar disorder and Diabetes mellitus. Non-adherence to medication was mainly non-intentional in subjects with diabetes mellitus (67.5%) while it was mainly intentional in subjects with Schizophrenia (69.9%) and Bipolar disorder (71.4%). Conclusion: This study has shown that poor adherence to treatment and its implication for treatment in chronic illnesses like Schizophrenia, Bipolar disorder, Diabetes mellitus remains a serious problem in this environment and continues to be so because of such risk factors as non supervision of drug intake, multiple number of tablets prescribed in a day and issues of cost of medications identified in this study. Concerted effort of all stake holders in patient’s management will be required to implement simple interventions/counselling measures to ameliorate these factors and ultimately improve treatment adherence and outcome in subjects with chronic illness in this e