Aldosterone-producing adenoma (aldosteronoma)--the most frequent form of primary hyperaldosteronism (PH)--is considered a specific form of diabetes mellitus (DM). In a previous study we demonstrated insulin resistance in patients with PH. We have therefore undertaken a study to evaluate the incidence of abnormalities of glucose metabolism in patients with PH (36 subjects) compared to control subjects with essential hypertension (EH) (21 patients). The following parameters were measured in all studied subjects: office blood pressure (by mercury sphygmomanometer in the sitting position), body mass index (BMI), plasma potassium, plasma glucose and insulin levels during oral glucose tolerance test (OGTT) (0, 60, 120 min), plasma renin activity and plasma aldosterone. Although patients with PH tended to have higher stimulated plasma glucose levels after 60 and 120 min compared to EH, these differences did not attain statistical significance. Patients with EH tended to have higher insulin levels at each measured interval, but due to a high variability these differences were again not significant. There were no significant differences between PH and EH in the proportion of diabetics (20% vs. 14%) or patients with impaired glucose tolerance (18% vs. 10%). In conclusion, we have found the absence of significant differences in the frequency of diabetes mellitus, impaired glucose tolerance and insulin resistance in patients with EH and PH. Our data thus do not support the idea of primary hyperaldosteronism as a specific type of diabetes mellitus. Furthermore, our results indicate that glucose metabolic characteristics in essential hypertension and primary hyperaldosteronism tend to be similar. The definitive conclusion with respect to the possible causal relationship between DM and PH, however, can be obtained only on larger groups of subjects, in particular after the evaluation of the effect of surgical/pharmacological treatment of primary hyperaldosteronism.