Our objective was to determine whether preterm birth of a singleton is associated with an increased risk of preterm birth of twins in a subsequent pregnancy. We identified all women who delivered a singleton followed by twins at Northwestern Memorial Hospital during a 10-year period. Using a cohort study design, we compared women with preterm singleton deliveries to women with term singleton deliveries with regard to their subsequent twin pregnancy outcomes. Two hundred ninety-three were identified who delivered a singleton followed by twins. Women who delivered a preterm singleton were significantly more likely to deliver subsequent preterm twins (73.9% versus 44.4%, odds ratio 3.5, 95% confidence interval 1.4 to 9.3). This significant difference persisted in multivariable analysis after controlling for ethnicity (adjusted odds ratio 3.3, 95% confidence interval 1.3 to 8.7). We concluded that preterm birth of a singleton is associated with an increased risk of preterm delivery in a subsequent twin gestation.