The Sydney Lead Study is a prospective investigation of the relationship between low-level lead exposure and neurobehavioural development during the first five years of life. Of the initial cohort of 318 children, 207 remained at the end of the fourth year. Average blood lead levels at 42 and 48 months were 10.7 and 10.1 micrograms/dl, respectively, with only a minority of observations exceeding 15 micrograms/dl. The regression analyses support earlier findings from the study, in that exposure to lead resulting in the range of blood lead levels found in this cohort is not associated with mental or motor deficits in the preschool years.