We compared the effects of olive oil rich in oleic acid (chain length [C], 18 carbon atoms, with 1 double bond [18:1]), and sunflower oil rich in linoleic acid (C 18:2) in free-living nondiabetic patients with arterial hypertension fairly well controlled with antihypertensives. To our knowledge, this is the first study investigating long-term effects of MUFA or PUFA on BP. A 6-month treatment period was chosen to overcome the problem of a crossover effect of the previous treatment. In fact, although drug administration was similar after 2 months of either dietary regimen, a marked reduction in hydrochlorothiazide (−65 mg/d), nifedipine (−75 mg/d), and atenolol (−175 mg/d) dosage was observed after 4 months of olive oil intake, becoming even more marked at the end of the study. Moreover, tests for treatment-period interaction clearly demonstrated that olive oil was more effective in controlling BP values independently of the sequence. During the observation period, the variation in dietary components was lower than expected in a free-living population. As a matter of fact, the diet prescribed was similar in both treatment periods, with the only difference confined to the investigative factor, ie, extra-virgin olive oil vs sunflower oil. Moreover, patient compliance to the diet was strictly controlled by a dietitian, who reinforced the advice during the single interviews with the participant. The main result of our investigation was a straightforward reduction in antihypertensive tablet consumption when patients were given olive oil, whereas drug consumption was only mildly affected by sunflower oil. At variance with experimental diets, free diets were higher in total (34.0% vs 26.6%; P = .005) and saturated (11.0% vs 5.6%; P<.001) lipid content. This is in line with results from epidemiological studies indicating that the rural habits of the traditional Mediterranean diet typical of southern Italy probably have changed since the original observations of the Seven Countries Study23,24 to resemble a more continental diet, ie, one richer in saturated fats.25 A slightly favorable effect of unsaturated fat on serum lipid levels has also been detected, since we found a slight reduction in serum total cholesterol (−0.336 and −0.233 mmol/L [−13 and −9 mg/dL] during the MUFA and PUFA diets, respectively) and triglyceride levels (−0.215 and −0.068 mmol/L [−19 and −6 mg/dL] during the MUFA and PUFA diets, respectively). The between-group difference did not reach statistical significance. The magnitude of this effect was probably minimized by the characteristics of the patients, most of them having cholesterol and triglyceride levels within normal limits. The pressor effect of oleic acid is independent of weight loss and other possible confounding variables that might affect BP levels (ie, potassium level).10 The mechanisms behind the BP reduction induced by olive oil are not easily understood. It has been shown that insulin sensitivity increases when the polyunsaturated-saturated fat ratio in dietary intake increases. Accordingly, it has been shown that the decrease in the PUFAs of skeletal muscle phospholipids is associated with reduced insulin sensitivity.26 We know that insulin resistance, and the consequent hyperinsulinemia, are well related to arterial hypertension; however, this does not seem a likely explanation for our finding. In fact, it would be difficult to explain why different fatty acids of the same chain length act differently on BP control, since insulin resistance is inversely related to total percentage of C 20-22 PUFAs.26 The unsaturated fatty acids are also able to reduce serum levels of the vasoconstrictor thromboxane 2, which might influence BP regulation. Again, this does not seem a likely explanation for our finding, since oleic acid is not expected to influence this variable more favorably than polyunsaturated linoleic acid.