Adolescent pregnancy and adolescent drug use are important clinical and public health problems. Yet, few studies have systematically investigated the patterns of substance use among pregnant and parenting adolescents. Because adverse outcomes are not found uniformly for all adolescent mothers, use of illicit drugs may be a key factor in determining which mothers and their infants will have poor outcomes. In this study, the patterns of drug use are described and differences in the demographic and psychosocial profile among 253 pregnant adolescents are investigated. Results obtained from interviews and urine assay for marijuana and cocaine indicate that lifetime use was 84% for alcohol, 62% for marijuana, and 23% for cocaine, whereas use in the past year was 40% for marijuana and 17% for cocaine. Compared with nonusers, pregnant adolescent drug users were more likely to be North American black, have a history of elective abortion and venereal disease, report more negative life events and violence during pregnancy, and receive more support from the father of the baby who was more likely to use marijuana and cocaine (P less than .01). Furthermore, according to logistic regression analysis results after controlling for age and ethnicity, adolescents who used illicit substances in the past year were three times more likely to have a male partner who used marijuana or cocaine and were two times more likely to have a history of venereal disease compared with nonusers. The findings suggest that drug use, whether as a mechanism or a marker, is associated with social and medical characteristics that are likely to contribute to negative outcomes among adolescent mothers and their infants.(ABSTRACT TRUNCATED AT 250 WORDS)