Objective To determine the risk of metabolic syndrome (MS) and polycystic ovary syndrome (PCOS) in a cohort of indigenous South Asian women with a recent history of gestational diabetes mellitus (GDM).Design Case-control study.Setting Department of Obstetrics & Gynaecology, University of Colombo, Sri Lanka.Sample Two hundred and seventy-four indigenous Sri Lankan women with previous GDM and 168 ethnically matched controls. Of these, 147 with previous GDM and 67 controls not taking hormonal contraception participated in an in-depth endocrine study.Methods Assessing the prevalence of MS and PCOS based on clinical features, biochemistry and ovarian ultrasound examination at 3 years postpartum. Main outcome measures Prevalence of MS and PCOS.Results Women with previous GDM and controls were studied at a mean (range) of 34.6 (13.4-84.1) and 46.5 (17.5-78) months postpartum, respectively. Those with previous GDM had a larger mean ± 95% confidence interval waist circumference (90.9 ± 1.3 versus 81.2 ± 2.8 cm, P = 0.0004) and were more likely to have hypertension (17.6 versus 7.4%, P = 0.001), glucose intolerance (51.7 versus 10.4%, P = 0.00001), hypertriglyceridaemia (16.3 versus 5.9%, P = 0.02) and a lower level of high-density lipoprotein (70 versus 56.7%, P = 0.04) than the controls. Of the women who had GDM, 72 (49%) had MS, 86 (58.5%) had polycystic ovaries and 59 (40%) had PCOS, significantly more than the control women-4 (6%), 9 (13%) and 2 (3%), respectively (P = 0.00001).Conclusions The prevalence of MS and PCOS in indigenous Sri Lankan women 3 years postpartum is significantly higher in those with previous GDM compared with ethnically matched controls. This confirms an association between GDM and subsequent PCOS and MS.