Dyslipidaemia is a common finding in the syndrome of gestational diabetes mellitus (GDM). Free fatty acids (FFA) can cause insulin resistance and have a toxic effect on beta cell function. GDM is a syndrome due to insulin resistance and the purpose of this study was to examine FFA levels in pregnancy. For this purpose samples were taken from 44 consecutive women with GDM and 36 consecutive controls. The women with GDM had similar levels of insulin, triglycerides and total cholesterol to the controls but a significant elevation in FFA; 0.70 (0.33) mmol/L versus 0.29 (0.12) mmol/L (p < 0.0001). A raised level of FFA may be responsible for the insulin resistance of GDM and may assist in the diagnosis.