To assess the extent of prenatal substance use in a predominantly white population in an urban area of the western United States and to develop a risk profile for this population, a cross-sectional prevalence study was done. Prenatal clinics (10 public and 10 private) anonymously recorded demographic information about and collected aliquots of routinely obtained urine specimens from women during prenatal visits. Urine specimens were screened by enzyme immunoassay for amphetamines, marijuana, cocaine, opiates, and ethanol. Of the 935 women screened, 92 (9.8%) had urine specimens positive for one or more of these substances. Urine screens were positive in 56 (10.0%) of 562 women attending private clinics and 36 (9.6%) of 373 women attending public clinics. Only 7 of the 935 women (0.7%) had screens positive for cocaine. Ethanol was the most frequently detected substance in the private clinic group (6.4%), whereas marijuana was most common among women attending public clinics (5.1%). Although substance use in this group of pregnant women occurs at a lower rate and a different pattern from those found in other more densely populated areas, the rate is high enough to be of concern to all prenatal care professionals, who should incorporate substance use history taking and selective urine drug screening into their routine prenatal practices.