This case-control study tested the hypothesis that pregnant inner-city women with low utilization of prenatal care are likely to be frequent drug users. Cases registered consecutively for prenatal care at > or = 28 weeks gestation or had < 4 prenatal visits. Controls were matched to cases by date of delivery. 24/81 (30%) cases and 16/128 (12%) controls were frequent drug users (adjusted odds ratio = 2.5; 95% CI, 1.2-5.4). Drug use (P = 0.01) and socioeconomic status (P = 0.001) were significantly correlated with prenatal care utilization. Self-report alone failed to note as many drug users as toxicology screen alone. Both substance use history and toxicology screen are advisable in women with low utilization of prenatal care.