Serum calcium concentration has recently been shown to predict cardiovascular mortality in a large health-screening program. Since impaired glucose tolerance (IGT) is an independent cardiovascular risk factor, we examined the association between glucose intolerance and serum calcium in a population-based cohort study. To characterize this association, we measured total serum calcium, parathyroid hormone (PTH), 25-hydroxyvitamin D (25OHD), and 1,25-dihydroxyvitamin D (1,25-(OH)2D) levels in a cohort of 1,071 randomly selected white individuals aged 40 to 65 years in whom an oral glucose tolerance test had been completed. In multivariate analyses, the 2-hour plasma glucose was positively associated with increasing total serum calcium and PTH in men and women after adjustment for age, obesity, season, and 25OHD. The adjusted odds ratio (OR) between increasing quintiles of total serum calcium and IGT was 1.63 (95% confidence interval [CI], 1.42 to 1.88). The OR comparing the top with the bottom quintile was 8.5 (95% CI, 4.5 to 16.0). The association with quintile of serum PTH was 1.30 (95% CI, 1.14 to 1.49). These data suggest that IGT is associated with an increase in both total serum calcium and PTH that cannot be explained by confounding by aging, obesity, or 25OHD. This relationship may explain the previously observed association between serum calcium and cardiovascular mortality. Whether this association is a manifestation of a shared cellular defect or represents a common relationship with an unknown etiologic factor are important questions for further research.