1. The association between vegetarianism and blood pressure was studied in relation to obesity, sex and aspects of lifestyle in 180 Seventh-day Adventists and 113 Mormons aged 25-44 y. 2. Volunteers completed a questionnaire, a 1-day diet record and submitted to standardized measurements of blood pressure, heart rate and body size. 3. Ninety-eight Adventist "vegetarians' were comparable to the 113 Mormon omnivores for strength of religious affiliation, consumption of alcohol, tea and coffee and use of tobacco, but were significantly less obese. 4. Obesity correlated positively with blood pressures in males and females of both diet classes. Age showed a positive correlation with blood pressure in females only. 5. Adjustment of blood pressures for age and Quetelet Index indicated that there is an additional blood pressure reducing effect associated with a vegetarian diet.