We used a prospective study design to assess the effects of prenatal low-level lead exposure on the development of urban, inner-city children in Shanghai. Umbilical cord blood samples wee consecutively collected from 605 live newborns. Two hundred and fifty-seven samples were excluded from the study due to clotting. Lead levels were determined on 348 cord blood samples. The geometric mean was 9.2 micrograms/dl. Based on their cord blood lead levels, infants were classified into two exposure groups: 104 in a relatively low lead group (lead levels < or = 30 percentile), and 104 in a relatively high lead group (lead levels > or = 70 percentile). Seventy-five subjects failed to complete the study, and 133 babies were included in the final cohort: 69 babies in the high lead group and 64 in the low lead group. At 3, 6, and 12 months, the Bayley Scales of Infant Development were administered and capillary blood lead levels were measured. Detailed information was obtained on a wide range of variables relevant to infant development. At all three ages, the Mental Development index (MDI) scores, adjusted for confounders, were inversely related to the infants' cord blood lead levels. The difference of the mean adjusted MDI scores between low and high lead groups was 3.4 at 3 months, 6.3 at 6 months, and 5.2 at 12 months of age. These differences were statistically significant at all time points. No significant association between cord blood lead levels and the Psychomotor Development Index (PDI) scores was detected at all three visits after adjustment for confounders. Postnatal lead levels were unrelated to concurrent developmental status. We conclude that prenatal low-level lead exposure, which is relatively common in Shanghai, is associated with an adverse developmental impact on children through the first year of life.