The incidence of gestational diabetes in the Illawarra area is 7.2% (95% CI: 6.0-8.4). This was determined by data collected by 2 prenatal clinics and 3 private practitioners during 1993. The incidence rate varied between 5.1% at one hospital to 11.3% with a private practitioner. This variation was mainly due to differences in age and ethnic background. The highest incidence rate of 11.9% was found in women of Asian background.