Ninety-eight confirmed adult vegetarians were examined against a matched group of nonvegetarians living in the same urban environment in order to evaluate the prevalence of arterial hypertension. The average blood pressure was 126/77 for the vegetarians and 147/88 for the control group (p less than 0.05). Significantly lower blood pressure was found in every decade of age. Only 2% of the vegetarians had hypertension (higher than 160/95) as compared to 26% hypertensives in the nonvegetarians. These differences in blood pressure were maintained also when individuals with the same "relative weight" were compared. Family history of hypertension was similar in both groups. Analysis of factors such as coffee drinking and smoking did not favor reduced blood pressure among the vegetarian group. Sodium and potassium intake were evaluated from their ratios to creatinine in a single urine sample. It was evident that both groups excreted the same amounts of sodium, while potassium excretion was significantly higher in the vegetarians. In view of the increasing evidence that potassium plays an important role in the regulation of blood pressure it is concluded that the protective antihypertensive factor in the vegetarian diet is the presence of high amounts of potassium.