Summary. This study was conducted to understand the pathogenetic mechanisms that are involved in the development of bone loss in children with severe haemophilia A (HA). Fourty-four children with severe HA and 40 age- and gender-matched healthy control subjects were enrolled in this study. Markers of bone remodelling and osteoclast regulation including serum bone specific alkaline phosphatase, parathormone, 25-hydroxy-vitamin D3 (25HOvitD3), osteocalcin and calcitonin levels were studied. Bone mineral density (BMD) was also studied in all children. The measurement of markers of bone remodelling and osteoclast regulation suggested increased osteoclast-mediated resorption activity in children with severe HA. Although serum parathormone levels were significantly increased, serum 25HOvitD3 and osteocalcin levels were significantly reduced. BMD was significantly reduced in severe haemophilics compared with healthy controls. There was also significant inverse correlation between BMD z-score and total joint scores, and insignificant inverse correlation between BMD z-score and single joint scores. There were also significant inverse correlation between 25HOvitD3 and osteocalcin levels and total joint scores. Children with severe HA could have significantly reduced BMD, compared with gender- and age-matched healthy control subjects. Our results of the markers of bone remodelling and osteoclast regulation suggested that increased osteoclast-mediated resorption and decreased osteoblastic activity in children with severe HA. All children with severe HA should be routinely screened in terms of BMD.