Determinants that affect bone density in disabled children and adolescents with neuromuscular disorders have not been differentially investigated well. We performed dual-energy X-ray absorptiometry of the lumbar spine in three groups (Duchenne muscular dystrophy, n = 16; other neuromuscular diseases, n = 11; and cerebral palsy, n = 18) providing height-age- and sex-adjusted z scores. Mobility was assessed by functional tests. Seven Duchenne patients were taking glucocorticoids; two reported previous treatment. We documented vitamin D blood levels and markers of bone turnover. Many patients presented low bone density for height-age (mean z score = − 0.86 ± 1.47). Areal bone density increased with age in the cerebral palsy and “other neuromuscular disease” groups, however, the Duchenne group demonstrated a decrease of z scores (r = − 0.622, p = 0.010). Tanner stage, body mass index, and mobility were independent variables affecting bone density. Vitamin D levels were low, but similar to those reported in healthy children. We conclude that bone mineral density in disabled children is mainly determined by their level of physical maturity, thriving, and mobility.