INTRODUCTION: Serotonin receptors are present in osteoblasts and osteoclasts, and serotonin affects bone metabolism. The association of plasma serotonin with markers of bone formation and bone resorption in hemodialysis patients was evaluated.MATERIALS AND METHODS: Twenty-four hemodialysis patients (11 diabetics) and 22 healthy volunteers were enrolled into the study. Serotonin was assessed in platelet-free plasma, whereas the markers of osteoblastic activity N-terminal midfragment osteocalcin and total procollagen type-1 aminoterminal propeptide as well as the marker of osteoclastic activity beta-isomerized C-terminal cross-linked peptide of collagen type I were measured in serum. Serum intact parathyroid hormone was also assessed.RESULTS: Serotonin did not significantly differ between hemodialysis patients and healthy volunteers. All evaluated markers of bone metabolism and intact parathyroid hormone were much higher in hemodialysis patients. Serotonin was significantly correlated with all evaluated markers of bone metabolism in hemodialysis patients. Serotonin was reversely related to the patients' age. Serotonin, osteocalcin, procollagen type-1 aminoterminal propeptide, and beta-isomerized C-terminal cross-linked peptide of collagen type I were much lower in diabetic hemodialysis patients.CONCLUSIONS: Serotonin may increase both bone formation and bone resorption in hemodialysis patients. The reverse relation of serotonin to patients' age as well as its lower levels in diabetic hemodialysis patients indicate that low plasma serotonin may contribute to the higher incidence of low-turnover bone disease that characterizes old and diabetic hemodialysis patients.