Age-standardized estimates of prevalence of diabetes mellitus and impaired glucose tolerance (IGT) are presented for women aged 20-39 years in 29 populations. In most cases, diabetes was uncommon ( 10% in Nauruans and > 20% in Pima Indians. IGT was common (> 3%) in many communities. Prevalence of IGT exceeded 10% in black and Hispanic women in the United States, urban Indian women in the United Republic of Tanzania, Pima Indians and some Pacific populations. In one-third of the populations, prevalence of total glucose intolerance (diabetes and IGT combined) exceeded 10% in women in this age range. In many communities, over 50% of diabetes was undiagnosed prior to the survey. It is concluded that a substantial proportion of abnormal glucose tolerance during pregnancy will go undetected in the absence of screening programmes. An international study is required to define the risk to the mother and the fetus of various levels of glucose intolerance, with a view to formulating international guidelines for clinical practice.