['To investigate serum levels of the advanced glycation end-products (AGEs) pentosidine and N epsilon-carboxymethyllysine (CML) in patients classified into different osteoporosis subgroups according to histomorphometric data.', 'Serum samples were obtained from 116 osteoporotic patients (34 men, 82 women) classified by bone histomorphometry into subgroups with high turnover (HTO, n = 32), low turnover (LTO, n = 39), normal turnover (NTO, n = 9) and cellular uncoupled osteoporosis (CUO, n = 36). Pentosidine was measured by high-performance liquid chromatography, and CML by a competitive enzyme-linked immunoassay.', 'The entire osteoporosis group had significantly higher pentosidine and CML serum concentrations than healthy subjects. In contrast to healthy subjects, no correlation between levels of AGEs and age could be found. In subgroups characterized by increased bone resorption (HTO, CUO), serum pentosidine correlated significantly with the histomorphometric marker reflecting osteoclast activity/bone resorption (eroded surface as a percentage of trabecular surface). Moreover, in CUO a strong correlation between pentosidine and the mineral apposition rate was found. Surprisingly, in HTO the levels of CML and percentage of eroded surface were significantly negatively correlated.'] Are advanced glycation end-products pentosidine and N epsilon-carboxymethyllysine elevated in serum of patients with osteoporosis?