The health aspects of the use of lead in petrol were evaluated in the 1920s in the United States of America and, in spite of warnings from certain lead-toxicity experts, lead addition to petrol became standard international practice. Available data now show that lead in petrol at the scale of use in the 1970s produced significant environmental lead contamination and increased average blood-lead levels in the general population. National sample surveys of blood-lead levels in the United States carried out annually from 1976 show a decreasing trend closely correlated with the use of lead in petrol. Recent longitudinal epidemiological studies have concluded that the exposure levels associated with lead in petrol can cause a reduced average mental ability in children. These studies accounted for the potential confounding from socioeconomic and other factors. The practical conclusion from the studies reviewed is that there should be as little human lead exposure as possible, because there may be no threshold for the effects occurring and many thousand children have already been affected in the United States and other countries. The environmental health calamity caused by lead in petrol could have been avoided if the initial warnings had been heeded and better preliminary research of the health issues had been carried out. Nevertheless, incontrovertible proof of causality should not be required before regulations are made to protect public health.