On the basis of an emerging literature documenting associations of HO-1 with hyperglycemia, impaired glucose tolerance and T2DM, we used serum specimens gathered as part of a prospective cohort study of women receiving prenatal care before 20 weeks gestation to examine the association between early-pregnancy serum HO-1 concentrations and subsequent risk of GDM. The gene discussed is HMOX1; the disease is type 2 diabetes mellitus.