Babies born to teenagers aged 15-19 years have a substantial risk of dying within the first year of life. Although associated socio-demographic factors may account for an increase in the risk of adverse reproductive outcomes for teenagers, there is a concern that young maternal age may also be a biological risk factor. We examined the effects of maternal age of primiparous black women on the incidence of preterm births using data from 6,072 black women delivering between 1989 and 1995 at an urban perinatal network of 17 hospitals and health centres serving residents in a well-defined geographical area. Maternal age was grouped as: < or = 15, 16-17, 18-19, 20-24, 25-29 or > or = 30 years age groups. The 20-24 age group with the highest number of births and lowest preterm rate was used as the reference age. Preterm birth was defined as delivery < 37 completed weeks of gestation. Of the 6,072 infants born to the cohort, 1,170 (19.3%) were preterm. The unadjusted odds for a preterm birth for the < or = 15-year-olds (odds ratio [OR] = 0.97; 95% confidence interval [CI], 0.69,1.36), for the 16- to 17-year-olds (OR=1.21; CI=0.94, 1.57) and for the 18- to 19-year olds (OR=1.15, CI = 0.92, 1.43) were not significantly different from that for the reference group. The risk for the 25-to 29-year-old mothers was 1.26 times [CI = 1.05, 1.50] and for the > 30-year-old mothers 1.28 times [CI = 1.07, 1.52] that for the reference group. Adjustments using logistic regression analysis for the effects of maternal smoking, drug abuse during pregnancy, insurance status, having prenatal care and median family income from census tract of residence did not result in a significantly increased risk for preterm birth or low birthweight for the teenage groups compared with the reference group. We conclude that primiparous teenage black mothers do not have an inherent biologically increased risk for preterm births.