OBJECTIVE: To determine the prevalence of and risk factors for preterm labor (< 37 weeks of gestation) among young pregnant women in Brazil. METHODS: A national cross-sectional study was conducted among parturient women aged 15-24 years attending Brazilian public maternities in 2009. Questionnaires were administered by interviewers during parturient admission to the maternity clinic (or soon after delivery, if survey participants were in too much pain to respond during labor). The questionnaires collected demographic, behavioral, and clinical data. RESULTS: A total of 2 071 (86.3%) of the 2 400 parturient women invited to participate were included in the study. Mean age was 20.2 years (standard deviation [SD]: 2.7) and mean years of formal education was 8 (SD: 2.4). Preterm labor occurred in 450 women (21.7%). The highest proportion of premature labor (36.1%) was found in the North region and the lowest proportion (6.9%) was found in the South region. In the final multivariate logistic regression model, living in the North region (odds ratio [OR]: 1.43, 95% confidence interval [CI]: 1.28-1.59); having monthly income less than twice the Brazilian minimum wage (OR: 1.27, 95% CI: 1.01-1.60); having suffered domestic violence (OR: 2.27, 95% CI: 1.23-4.18); and having had inadequate prenatal care (OR: 3.17, 95% CI: 2.54-3.97) remained significant correlates of preterm labor in this population. CONCLUSIONS: Results show a high prevalence of preterm labor among young women in Brazil. Preterm labor has multifactorial etiologies. More public health resources must be directed toward accelerating understanding of this complex process.