It has been suggested that the incidence of gestational diabetes (GDM) may vary with different seasons of the year. The fasting and 2 h glucose levels after a 75 g glucose tolerance test were therefore examined in 2749 women being tested for GDM and were correlated with the mean temperature at 0900 h by month and by season. The mean monthly temperature ranged from 22.3 degrees C in January to 13.6 degrees C in July. The monthly incidence of GDM ranged from 3.5% (95% CI, 0.7-6.3) in September to 8.7% (4.7-12.7) in November. The seasonal incidence of GDM ranged from 7.4% (5.6-9.2) in autumn to 5.5% (3.5-7.5) in winter. Chi-squared analysis did not suggest a significant association between the proportion of women diagnosed with GDM and either the month (p = 0.47) or the season (p = 0.39). Multiple regression analysis, after correction for maternal age, parity, BMI, and the week of testing revealed a non-significant association between fasting glucose and monthly temperature (p = 0.36) and an increase in the 2 h glucose of 0.026 mmol l-1 for each increase of 1 degree C in temperature (p = 0.011). In a temperate coastal area of Australia these differences do not appear clinically significant and have not caused a variation in the incidence of GDM with different seasons.