The purpose of this study has been to establish the incidence of gestational hypertension (GH) in women with gestational diabetes mellitus (GDM) and to examine the frequency of complications in women with co-existent GDM and GH. Furthermore, we wished to evaluate the significance of urine albumin excretion determined by the urine albumin creatinine ratio (ACR). A total of 215 successive pregnancies with risk factors for gestational diabetes, as defined by the Danish National Board of Health screened for gestational diabetes by the WHO criteria, were reviewed. Women who had a normal OGTT during the screening served as controls. Gestational hypertension was defined as a mean arterial pressure (MAP) >105 mmHg (systolic BP > or = 140 mmHg and/or diastolic BP > or = 90 mmHg). The two groups were comparable with regard to initial body mass index (BMI) and MAP. GH appeared with a higher frequency in women with GDM (28%) than in women with normal OGTT (10%) P=0.003 chi(2) test. Serious complications (perinatal mortality, malformations, acute caesarean section) also appeared with a higher frequency in women with GH and GDM (10%) than in women with GH but with normal OGTT (2%) P=0.0083 chi(2) test. We observed a significant increase in ACR in the group with complications (GDM and GH) during gestation regardless of intensive antihypertensive treatment. We also observed that ACR was significantly higher in women with GDM and GH when compared to women with GDM and a normal blood pressure. The BMI was consistently higher in women with GH, regardless of whether they had GDM or not as compared to the normotensive group. GH appears with a higher frequency in women with GDM and the co-existence seems correlated with a higher frequency of complications. The correlation between urine albumin excretion and complications might suggest that regulating GH should strive to normalise ACR in women with GDM.