Background: A blood pressure below or equal to 140/90 mmHg, the aim of antihypertensive treatment, is rarely achieved. Only 16% of patients controlled by our group reach this goal. Aim: To analyze the causes of suboptimal treatment and to assess the effects of an optimization of antihypertensive therapy. Patients and methods: A random sample of 160 patients was analyzed and followed during one year. Results: Sixty six patients (41%) had a normal blood pressure, maintained during the first three months of follow up. The main causes of suboptimal reduction of blood pressure in the remaining 94 patients were an incorrect prescription or dosage of medications in 37.5%, lack of compliance in 34%, insufficient delivery of medications by the health service in 24% and secondary effects of drugs in 5%. When these factors were corrected, blood pressure normalized in 41 of them. In other 37, a reduction of 5 mmHg or more in blood pressure, was obtained. The most frequent changes introduced were modifications in dosage and addition of a new medication. Therefore, in 90% of these patients, blood pressure was reduced or normalized. Conclusions: A correct identification of the cause of antihypertensive treatment failure is imperative. The correction of this cause leads to a further reduction in blood pressure in 90% of those subjects with suboptimal treatment (Rev Méd Chile 2002; 130: 519-26)