Premature labor is a recognized complication of substance abuse in pregnancy. However, the prevalence of substance abuse in patients first seen with symptoms of preterm labor and who deny use of illicit drugs is unknown. Between July 1, 1988, and December 15, 1988, 141 patients first seen at Northwestern Memorial Hospital in suspected preterm labor and a comparison group of 108 uncomplicated laboring term patients had urine toxicology screening performed to detect substance abuse. Positive urine toxicology screenings were found in 24 of 141 (17.0%) patients with suspected preterm labor and in 3 of 108 (2.8%) patients in the comparison group. The most common substance identified was cocaine (14/24 and 1/3, respectively). Of those patients in suspected preterm labor, 22 of 86 (25.6%) clinic patients had positive urine toxicology screenings as did 2 of 55 (3.6%) private patients. On the basis of our observations, we recommend that patients admitted in possible preterm labor be encouraged to submit to screening for substance abuse so that appropriate counseling and prenatal and neonatal care become available for these high-risk patients.