Gestational diabetes mellitus (GDM) was diagnosed in 1928 of 35,253 (5.5%) tested pregnancies at the Mercy Maternity Hospital in Melbourne between 1979 and the end of 1988. Compared with women born in Australia and New Zealand, the incidence of GDM was significantly greater in women born on the Indian subcontinent (15%); in women born in Africa (9.4%), Vietnam (7.3%), Mediterranean countries (7.3%), and Egypt and Arabic countries (7.2%); and in Chinese (13.9%) and other Asian (10.9%) women. There was no significant difference for women born in the United Kingdom and northern Europe (5.2%), Oceania (5.7%), North America (4.0%), or South America (2.2%). With the World Health Organization criteria as a guide to the severity of hyperglycemia, compared with mothers born in Australia and New Zealand, there were significant increases in the incidences of the more severe grades of GDM in parturients born in the Mediterranean region, Asia, the Indian subcontinent, Egypt, and Arabic countries. The incidence of GDM increased significantly in all racial groups, rising from 3.3% during 1979–1983 to 7.5% during 1984–1988.