OBJECTIVE We studied the incidence of postpartum diabetes after gestational diabetes mellitus and investigated biochemical and clinical predictors of postpartum diabetes. METHODS We monitored 174 women with gestational diabetes by performing oral glucose tolerance tests during pregnancy as well as 1, 2 and 5 years postpartum. Women who developed impaired fasting glucose, impaired glucose tolerance or diabetes were compared with women who remained normoglycaemic at 5 years. Insulinogenic index, disposition index and HOMA-beta cell index were used to assess beta cell function; insulin resistance was estimated by HOMA index of insulin resistance. RESULTS At 5 years postpartum, 30% of the women had developed diabetes and 51% some form of abnormal glucose tolerance. Women who developed diabetes had higher fasting glucose and HbA(1c) during pregnancy than those who remained normoglycaemic. They also had lower HOMA-beta cell index, insulinogenic index and disposition index than the normoglycaemic women. HbA(1c) and fasting glucose during pregnancy as well as the number of previous pregnancies and family history of diabetes were independent predictors of postpartum diabetes. HbA(1c) > or =4.7% (Swedish Mono S) or > or =5.7% (National Glycohemoglobin Standardization Program) and fasting blood glucose > or =5.2 mmol/l were associated with a four- to sixfold increased risk. CONCLUSIONS Among women with gestational diabetes mellitus, those at risk of future diabetes can be identified by HbA(1c) and fasting glucose values in the upper normal range during pregnancy. A family history of diabetes and previous pregnancies further increase this risk.