To assess the relevance of unrecognized hyperglycemia among high-risk subjects for developing diabetes a cross-sectional study was carried out. Subjects aged 40-75 years with (high-risk group) and without (control group) history of impaired glucose metabolism underwent a 2h-oral glucose tolerance test (OGTT). All individuals with diabetes diagnostic criteria and all controls with glucose abnormalities at OGTT were excluded. An individualized 48-h continuous glucose monitoring (CGM) calibrated by fasting plasma glucose was performed. The microdialysis-based biosensor recordings were computerized in order to identify continuous glucose profiles. Of the 121 monitored subjects, 104 were finally analyzed (56.7% female, 57.8 years, BMI=29.2, A1C=4.9%, HOMA index=2.5). Glucose profiles corresponded to 31 controls (29.8%), 32 high-risk individuals with normal OGTT (30.8%) and 41 (39.4%) with hyperglycemia at OGTT. The recordings defined as hyperglycemia (fasting >or=6.1 mmol/l, non-fasting >or=7.8 mmol/l) appeared during an average of 1.4h, 4.9h and 7.6h (3.9%, 13.9% and 19% of the CGM time), respectively. The highest percentage of impaired CGM registers corresponded to the fasting period. Nevertheless, the longest duration corresponded to the non-fasting period. The CGM evidenced a relevant degree of casual undetected hyperglycaemia among high-risk individuals.