It has been suggested that long term treatment with L-thyroxine could reduced bone mineral density (BMD). The purpose of this study was to determine whether BMD is decreased by L-thyroxine treatment in children. Dual energy X-ray absorptiometry (DEXA) was used to assess lumbar spine (L2-4) and femur neck BMD in 40 children aged 9-15 years, taking L-thyroxine (100 micrograms/m2/day) for a mean period of 1.45 +/- 0.60 years for colloid diffuse goiter. Patients were matched with controls for age, sex, weight, height and pubertal stage. BMD at both the femur neck and lumbar spine was not significantly different from that of the control group. No correlation was found between BMD values and TSH levels which is the index of tissue hyperthyroidism. BMD was also not correlated with duration of the therapy. Osteocalcin, alkaline phosphatase, calcitonin and parathormone levels were measured to asses bone turnover; none of them were significantly different from those of controls and they did not change during follow up. In conclusion we suggest that long-term L-thyroxine therapy in children has no adverse effect on BMD.