Study aim: To examine the effects of a walking programme on the bone mass density (BMD) in sedentary, thin women aimed at preventing bone losses.Material and methods: Twenty thin (BMI<20) women aged 22.0 ± 1.5 years volunteered to participate in the study. They were randomly assigned into the exercise (n = 10) or control (n = 10) groups, those from the experimental group being submitted to a training programme lasting two months. The programme consisted of 3 walking sessions per week, 30 min each, at 50 - 75% of maximal heart rate. Anthropometric measurements, bone mass density (by DXA) at the hip and lumbar spine (L2 - L4) and oestradiol concentration in serum (by radioimmunoassay kits) in the follicular phase were made before and after the training programme. The same diet was maintained throughout the study and was monitored by 7-day recalls.Results: The walking programme induced significant increases in BMD (by 5.2% in the hip site, p<0.001, and by 7.3% in the spine, p<0.05). Significant decreases were found in calcium concentration in both groups (by about 5%) and in phosphorus concentration in the experimental group (by about 16%). In the experimental group also the relative body fat content significantly decreased (by 7.7%).Conclusion: Walking exercise practiced for two months reduced the risk of bone loss by significantly increasing bone density.