The effect on blood pressure of an ovo-lacto-vegetarian (OLV) diet was assessed in a randomized controlled crossover trial. Fifty-eight mild untreated hypertensive subjects recruited from the Perth Centre for the 1983 National Heart Foundation (NHF) Risk Factor Prevalence Survey 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. A significant fall in systolic blood pressure, on average of the order of 5 mmHg, was associated with eating an OLV diet. Blood pressure change was unrelated to change in urinary sodium, potassium or body weight, but was related to initial blood pressures. Although an OLV diet may have an adjunctive role in control of mild hypertension, in view of likely problems with acceptability the dietary components responsible for the blood pressure changes need to be identified.