A large body of experimental data has shown that lead raises blood pressure and increases responsiveness to alpha-adrenergic agonists in rats and pigeons. These studies suggest the need to look for a similar relationship in humans. This paper examines the robustness of the previously reported association between blood lead and blood pressure in adult males. The association remains strong and is essentially unchanged in tests that include nutritional factors and demographic factors, alone or together, or in tests that include insignificant terms. The relationship was not confounded by age; it held for all adult men in the 20-45 age group, the 40-59 age group, and the 46-74 age group. Interaction terms for 25 30-year age groups (20-49, 21-50,. . . , 45-74) were all insignificant, indicating no difference in the relationship by age. The relationship is also robust to the inclusion of a time trend to account possible omitted time-varying factors, and it held in a model that controlled for possible site effects. Given the strong experimental evidence, the relationship is likely causal.