OBJECTIVE 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. METHODS 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. RESULTS 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. CONCLUSIONS AGE-modified proteins may be a cause of disturbed bone remodelling in osteoporosis. Our findings do not support the alternative hypothesis that increased AGEs in serum indicate only a more intensive releasing of AGEs in circumstances of increased bone resorption.