Perinatal illicit drug use was determined by anonymously testing the urine of 1111 inner-city parturients. Fourteen percent tested positive, 11.5% for cocaine. As determined by self-report or by positive test results, 17% used drugs. Twenty-eight percent of low birthweights and 20% of premature births in this group were attributable to drug use. Risk of prematurity was reduced by a factor of 2 to 3 with adequate prenatal care. These data suggest an enormous potential for improving perinatal health.