The effect of an ovo-lacto-vegetarian (OLV) diet on blood pressure was assessed in a randomized, controlled, crossover trial in 58 mild untreated hypertensive subjects recruited from the Perth Centre for the 1983 NHF Risk Factor Prevalence Survey. Subjects were randomly allocated to one of three groups; the first maintained their usual diet throughout 12 weeks; the other two were given an OLV diet for either the first or second 6 weeks of the 12-week trial. Introduction of an OLV diet was associated with a significant fall in systolic blood pressure, on average of the order of 5 mmHg, which was unrelated to change in urinary sodium, potassium or body weight. It was concluded that a vegetarian diet may have an adjunctive role in control of mild hypertension, but that in view of likely problems with acceptability those dietary components responsible for the blood pressure differences need to be identified.