Heavy licorice (glycyrrhizin) consumption has been associated with shorter gestation. The aim of the present study was to test whether this association also applies to preterm (<37 weeks) births. In 2000-2001, a sample of 95 Finnish women who delivered preterm singletons was compared with controls (n = 107) who delivered babies of normal gestational age in the same hospital. Glycyrrhizin intake was calculated from questionnaires containing detailed items on licorice consumption. Glycyrrhizin exposure was grouped into three levels: low (<250 mg/week), moderate (250-499 mg/week), and heavy (> or =500 mg/week). Heavy consumption versus a lower level of consumption was associated with a more than twofold increased risk of preterm (<37 weeks) delivery. The association was stronger when only the 40 births classified as early preterm delivery (<34 weeks) were included (odds ratio = 3.07, 95% confidence interval: 1.17, 8.05 for the fully adjusted model). In conclusion, heavy glycyrrhizin exposure was associated with preterm delivery and may be a novel marker of this condition.