All English-speaking and Spanish-speaking women who were willing to give informed consent were consecutively recruited in the Women's and Adolescents' Prenatal Clinics at Boston City Hospital from July 1984 to June 1987, yielding an interview sample of 412 adolescents. All participants were interviewed during the prenatal and immediate postpartum periods by one of five bilingual interviewers. The interviews used a closed-end, forced-choice format to elicit information on the use of cigarettes, alcohol (frequency and quantity), marijuana (frequency and quantity), cocaine (frequency), and other illicit psychoactive substances before and during pregnancy (frequency). Participants were also asked to furnish urine samples at the time of each interview, to be assayed for marijuana and prescription and nonprescription drugs. The study shows that a small percentage of pregnant adolescents with positive urine assay results for marijuana denied the use of marijuana, even though they had been told their urine would be tested for marijuana. A much higher percentage of pregnant adolescents with positive urine assay results for cocaine denied cocaine use. Significant underreporting of drug use in this sample raises questions about similar underreporting of drug use among adolescents in the general population. 2 tables and 12 references