Objective: The International Association of Diabetes and Pregnancy Study Groups (IADPSG) has proposed new criteria for the diagnosis of gestational diabetes mellitus (GDM). The aim of this study was to compare the prevalence of GDM when IADPSG criteria were used with the prevalence when the current Australasian Diabetes in Pregnancy Society (ADIPS) criteria were used. Design, setting and participants: This was a prospective study over a 6-month period, examining the results of all glucose tolerance tests (GTTs) conducted for the diagnosis of GDM in Wollongong, a city using the public and private sectors. Main outcome measures: The prevalence of GDM using the existing (ADIPS) and the proposed (IADPSG) criteria. Results: There were 1275 evaluable GTTs (571 public and 704 private). Using the current ADIPS diagnostic criteria, the prevalence of GDM was 8.6% (public), 10.5% (private) and 9.6% (overall). Using the proposed IADPSG criteria, the prevalence of GDM was 9.1% (public), 16.2% (private) and 13.0% (overall). Conclusions: The proposed IADPSG criteria would increase the prevalence of GDM from 9.6% to 13.0% (P <0.001). In our study in the Wollongong area, which has a population with a predominantly white background, this increase came mainly from older women attending a private pathology provider. Data from both the public and private sectors need to be included in any discussion on the change in prevalence of GDM.