To examine factors associated with preterm birth and low birthweight and the role of paternity status in birth outcomes among racial/ethnic groups in Milwaukee. Retrospective analysis of data on 151,869 singleton live births (1993-2006) from the City of Milwaukee, Wisconsin. Multivariate logistic regression models were used to examine demographic and medical factors associated with racial/ethnic disparities in preterm birth and low birthweight. African-Americans, whites, Hispanics, and women of "other" racial groups accounted for 46%, 33%, 16%, and 5% of births, respectively. Preterm birth and low birthweight rates were three times greater for African-American women compared to whites. Compared to white women, the odds of preterm birth were 82% and 35% greater for African-American and other minority women, respectively. All minority women had greater adjusted odds of low birthweight than whites, with African-American women at greatest risk (OR 2.36:2.23-2.49). Across racial/ethnic groups, significant predictors of both outcomes included being unmarried with no child's father on record, maternal smoking, chronic hypertension, previous preterm birth, and inadequate and adequate plus prenatal care. Paternity status had a gradient effect for whites and Hispanics with unmarried women with no child's father's name on record at greatest risk, followed by those with court-established paternity and those with paternity statement at lowest risk for both outcomes. Implementing policies/programs that promote smoking cessation, proper management of maternal conditions, targeted interventions for women with previous preterm birth, and paternal involvement have the potential to reduce disparities in birth outcomes.