The major determinants of blood-lead levels were studied in 90 children who attended an outpatient pediatric clinic in Mexico City. All children, who were from 1-10 y of age, were from homes for which socioeconomic status had been categorized as medium to high. Blood-lead levels ranged from 0.17 (standard deviation [SD] = 0.008) to 1.21 (SD = 0.06 mumol/l). The main determinant of blood-lead levels was place of residence. Children who lived on private streets (i.e., low-traffic areas) had a significantly lower blood-lead level than children who lived on large avenues and who resided close to main roads (p = .0001, r2 = .27). This observation documented high exposure levels among children who live in Mexico City and suggested that leaded fuel used in Mexico could play an important role in determining blood-lead levels in this population.