Non-adherence to medication is often an unrecognized risk factor that contributes to failure of the therapeutic plan. The purpose of the study was to identify factors related to high, medium and low medication adherence among adult Saudi patients with hypertension and diabetes mellitus. This study is designed as a descriptive cross sectional survey and was conducted in three tertiary care hospitals of Al-Qassim province of Saudi Arabia. The data was collected using the 8-item Morisky Medication Adherence Scale (MMAS-8) and analyzed by SPSS. Three levels of adherence were considered based on the following scores: 0 to <6 (low); 6 to <8 (medium); 8 (high). Of the 396 patients interviewed, 52% reported low adherence to prescribed medication. Multinomial logistic regression analysis was conducted. Gender, age, literacy level, duration of illness and type of chronic disease were negatively associated with medication adherence. The study shows very high proportion of low and medium adherence on long term medication, which may be responsible for the failure of achieving therapeutic outcome. Further investigation is required to evaluate the applicability of MMAS-8 as a tool of measuring medication adherence among Saudi patients with chronic diseases. Adherence enhancing strategies should also be evaluated in separate patients group.