Lactation is known to be associated with a transient loss of bone mineral density (BMD) during 3–6 months post-partum. Bone changes during lactation in women consuming low dietary calcium are not sufficiently studied. The present longitudinal study examined the BMD changes during lactation in undernourished women and the relationship of bone changes to the nutritional status. Whole-body bone mineral content and BMD at hip, lumbar spine and forearm were assessed using dual-energy X-ray absorptiometry in thirty-six lactating women from the low socio-economic group at four time points – within 1 month after delivery (baseline), and at 6, 12 and 18 months after delivery. Maternal body composition and biochemical parameters of bone metabolism were estimated at the same time. It was observed that femoral neck BMD reduced by 4·6 % at 6 months, but recovery to the baseline was incomplete at 18 months with a deficit of 2 %. Hip BMD reduction at 6 months was transient. Lumbar spine BMD did not show significant loss at 6 months and BMD increased by 3·6 and 6·3 % at 12 and 18 months, respectively. Regression analyses indicated that baseline lean mass was the most important determinant of bone preservation at femoral neck, hip as well as whole body, whereas baseline body weight was the most important determinant of per cent gain in lumbar spine. Maternal nutritional status as indicated by body weight and lean mass appears to influence the lactation-related BMD changes in undernourished women from the low socio-economic group in India.