We studied the relationship between the age of reaching motor milestones, especially anti-gravity activities, and the age of development of pathological spinal curvatures in children with osteogenesis imperfecta (OI). We hypothesized that earlier achievement of anti-gravity motor milestones predicts a later development of pathological spinal curvatures. Ninety-six children participated in this retrospective study. The severity of the disease was classified according to Sillence into types I-IV. Spinal radiography was performed annually and spinal deformities were measured according to the Cobb angle. Scoliosis was defined as a Cobb angle exceeding 9 degrees. Pathological thoracic kyphosis was defined as a Cobb angle exceeding 40 degrees. The parents were asked to report the age at which the child achieved motor milestones, and data were checked against health care records. Thirty-seven of 96 children (39%) developed a scoliosis of more than 9 degrees. Nine of 96 children (9%) developed a pathological kyphosis. The age of developing scoliosis was significantly lower than the age of development of the pathological kyphosis (P=0.01). Bone mineral density was measured by dual energy X-ray absorptiometry (DEXA) in 53 children, 28 of whom developed scoliosis, and 25 of whom did not. The mean DEXA Z-score of the 28 children with scoliosis was significantly lower than that of the 25 children without (-5.2, SD 1.3 vs -3.2, SD 1.9; P-value <0.001). Children with OI type IV, but particularly OI type III, reached motor milestones much later than children with OI type I. The motor milestone "supported sitting" showed a significant inverse association with time of the first presence of scoliosis with a Cobb angle greater than 9 degrees (linear regression coefficient: -1.3, 95% confidence interval: -2.6 to -0.03). The age of achieving the motor milestones "lifting the head to 45 degrees in prone position", "rolling", and "supported-" and "unsupported standing" were not significantly associated with age of the first presence of scoliosis with a Cobb angle greater than 9 degrees. However, the directions of associations suggest that here, too, there is a tendency for later development of scoliosis in those who reach milestones at earlier ages. Multivariable analyses showed that the motor milestone "sitting with support" was significantly associated with age of first achieving scoliosis, independent of gender and type of OI (linear regression coefficient: -0.9, 95% confidence interval: -1.3 to -0.5). We conclude that in children with OI, the age of anti-gravity motor milestones was associated with the age of development of pathological spinal curvatures. Earlier achievement of the motor milestone "supported sitting" predicted significantly a later development of pathological spinal curvatures, independent of gender and type of OI.