The effect on blood pressure of elevating the dietary polyunsaturated/saturated fat (P/S) ratio was assessed in a double-blind, randomized control trial. Fifty-four healthy, normotensive volunteers aged between 20 and 59 years were randomly allocated either to a control group who ate a low P/S ratio diet throughout, or to one of two experimental groups who ate a high P/S ratio diet for one of two six-week experimental periods. Other nutrient changes were avoided. Twenty-four-hour diet records showed substantial changes in the P/S ratio when on the high P/S ratio diet, and no change in the control group or either experimental group when on the low P/S ratio diet. Relative concentrations of linoleic acid in plasma and cheek cell phospholipids were significantly increased when on the high P/S ratio diet. Changes in blood pressure, before and after adjustment for other possible confounding factors, were not related to changes in P/S ratio. It was concluded that an increase in P/S ratio per se cannot account for the previously reported blood pressure lowering effect of a vegetarian diet.