Diabetes mellitus in nonpregnant adults is a chronic affliction that leads to significant vascular and neuropathic disease. Diabetes during pregnancy can lead to perinatal complications. Both of these types of diabetes are common, often asymptomatic, and readily diagnosable by glucose tolerance testing. As a result, screening can identify many previously undiagnosed patients. However there is only limited evidence that screening results in net therapeutic benefit. In the case of gestational diabetes, controlled trials indicate that hypoglycemic therapy decreases the frequency of macrosomia, but has no effect on perinatal mortality. Our analyses indicate that screening for gestational diabetes is a low-cost intervention that produces a small expected benefit. Screening for diabetes in the nonpregnant adult (almost always a type II diabetic) is not recommended, because the link between improving glucose control and reducing diabetic complications is currently too weak. Screening might be reasonable for particular patients, for example, obese persons who would be spurred to lose weight by a demonstration of glucose intolerance. Screening for type I diabetes followed by immunomodulating therapy is still too experimental for confident analysis.