OBJECTIVES: In a previous study, we have identified side effects and lack of drug insurance coverage as the determinants of discontinuation in a group of patients newly prescribed antihypertensive medications. The current study aimed at identifying the determinants of non adherence to medication among those who had not discontinued. METHODS: We conducted a prospective cohort study in which individuals prescribed a new antihypertensive monotherapy were identified through a network of 173 pharmacies. We interviewed participants by telephone three times over a 3-month period. At the end of this period, those individuals which reported still taking the medication initially prescribed, were included in the analysis. Self-reported non adherence was measured at three month using the Morisky's 4-item questionnaire. Those answering yes to any one of the 4 questions were deemed to be non adherent. We analyzed data using a multivariable logistic regression model. RESULTS: Of 509 eligible participants, 118 (23.2%) reported non adherence to their drug treatment. Non adherence was significantly associated with the use of angiotensine converting enzyme inhibitors (Adjusted Odds Ratio (AOR) = 3.0; 95% Confidence Interval (CI) 1.2-7.9) as compared to angiotensin II antagonist losartan, and the belief that hypertension is not a risk factor for cardiovascular diseases (AOR = 2.0; 95% CI 1.2-3.3). On the other hand, non adherence was inversely associated with the use of more than 4 pills of medication a day (AOR = 0.3; 95% CI 0.2-0.6). CONCLUSIONS: Our findings suggest that determinants of non adherence are not the same as those for discontinuation. They also suggest that adherence to drug treatment could be improved by a proper selection of medication, and by attempts to correct wrong perceptions patients may have about hypertension. Our finding that taking more than four pills was associated with better adherence is concordant with data recently published by others.