Bisphosphonates are now widely used to treat children with osteogenesis imperfecta (OI). However, there are few published data following the initial evolution of changes in bone mass with such treatment. The purpose of the present study was to assess indices of skeletal size and total and regional bone mass in children with OI during the first year of their pamidronate therapy. Twenty-six children [11 males, age median 10.6 (range 3.2-15.5) years] with type III (n=9) or type IV (n=17) OI received intravenous pamidronate 1.0 mg/kg/day on 3 consecutive days, 3-monthly over a 1-year period. Bone mass assessed by dual energy X-ray absorptiometry, serum alkaline phosphatase (ALP) and urine type I collagen N-terminal telopepetide (NTX) were measured at each cycle. Lumbar (L2 to L4) vertebral height increased by median (interquartile range) 4.5% (7.3), area by 11.3% (19.3), bone mineral content (BMC) by 73.7% (50.4), and bone mineral density (BMD) by 48.7% (34.7), P<0.0001 during the first year of therapy. Total body bone area increased by 26.7% (26.3), total body BMC by 41.4% (44.5), and total body BMD by 8.8% (8.3), P<0.0001. Head bone area remained constant whilst head BMC and BMD increased. Pre-infusion U-NTX and S-ALP levels decreased progressively during the follow-up indicating reducing bone turnover rate during pamidronate therapy. Increased bone area and mineral mass were observed during the first year of intravenous pamidronate therapy in children with OI. Increased head bone mass implies a direct effect of pamidronate on skeletal mass accretion, rather than an indirect effect mediated through increased physical activity.