The prevalence of gestational diabetes mellitus has been reported to vary widely in aboriginal populations. Most of the data have come from the United States. To help determine the extent of gestational diabetes in Canada's aboriginal population, the authors assessed the prevalence in a population of Cree women in northern Quebec. A cross-sectional study was conducted using the National Diabetes Data Group (NDDG) criteria. Information was obtained from patient charts on pregnancies between January 1995 and December 1996 among women residing in 9 Cree communities in the eastern James Bay region of northern Quebec. Women who were not Cree, had pre-existing diabetes, had spontaneous abortion or were receiving glucocorticoid treatment were excluded. Data on 654 pregnancies that met the inclusion criteria were available. Results of the screening oral glucose challenge test were available for 579 of the pregnancies; the remaining 75 were excluded. The mean gestational age at screening was 28.3 (standard deviation 2.6) weeks. The prevalence of gestational diabetes was 12.8% (74/579) (95% confidence interval [CI] 10.1%-15.5%). The prevalence in the inland communities was twice as high as that in the coastal communities (18.0% v. 9.3%, p = 0.002). Women with gestational diabetes or impaired glucose tolerance tended to be older, have had more pregnancies, weigh more before pregnancy and have heavier babies than those with a normal glycemic status. The prevalence of gestational diabetes among James Bay Cree women in northern Quebec is twice as high as that among women in the general North American population and the second highest reported in an aboriginal group worldwide.