Decreased bone mass in early childhood is an increasingly recognized problem in classical galactosaemia as in many other chronic diseases. Peak bone mass is reached in late adolescence; thus, increasing peak bone mass in childhood can prevent osteoporosis. Regular bone mass measurements and preventive treatment should begin in childhood. In the absence of evidence-based guidelines for identification and treatment of decreased bone mass in children, we provide a proposal based on our experience and the available literature. Dual-energy x-ray absorptiometry (DXA) should be used for bone mass assessment. Because cooperation is required, measurements can usually be performed from the age of 4 years. Interpretation of bone mass measurements is crucial for the diagnosis of osteopenia or osteoporosis. In children and adolescents, total body bone mineral content (BMC) as well as lean tissue mass (LTM) should be measured. Comparison of BMC corrected for LTM of the patient with the BMC corrected for LTM of healthy controls allows correction for the confounding effect of bone size. DXA should be repeated every two years in case of normal BMC, as this is the time window in which abnormalities become measurable. If BMC is between 0 and -1 SD, lifestyle factors such as physical activity, intake of calcium and vitamins K and D and oestrogen supplementation (in girls) should be optimized. If BMC is below -1 SD, we advise to start with supplementation of calcium, vitamin K(1) and vitamin D(3). DXA should be repeated yearly in case of BMC below 0 SD in order to identify deteriorations and improvements early.