The urinary delta-aminolevulinic acid (ALA) test has been described as a simple, dependable test for detection of lead poisoning. To establish the validity of this claim, 6,811 Chicago slum children were screened for lead poisoning, using both blood lead and urinary ALA determinations. Blood was drawn from all subjects, but urine was obtained from only 3,227 children. Blood lead values were determined with a Beckman atomic absorption spectrophotometer. Urinary ALA was isolated with a disposable chromatographic column. Elevated ALA was found in 193 children, of whom 33 (17%) showed concomitant elevation of blood lead. Of 3,227 children, 147 (4.6%) had increased blood lead values, i.e., 50 μg. per 100 ml. or greater; 114 (78%) of the 147 had normal urinary ALA. Forty-three of the 147 children with elevated lead values were treated with chelates; 29 (67%) of the 43 had normal ALA. It is concluded that urinary ALA is not a dependable indicator of lead poisoning.