Many children and young adults with cerebral palsy (CP) have diminished bone mineral density (BMD) and a propensity to fracture with minimal trauma. The aim of this study was to identify variables which are routinely assessed as part of standard clinical care and that might be used to identify those individuals with CP who are most likely to have low BMD. One hundred and seven participants (ages 2 years 1 month to 21 years 1 month; mean age 10 years 11 months, SD 4 years 2 months) with moderate to severe spastic CP were assessed in detail. This included gathering clinical data, taking anthropometric measures of growth and nutrition, as well as dual energy X-ray absorptiometry measures of BMD. Seventeen participants were ambulatory with assistance (Gross Motor Function Classification System [GMFCS] level III), and 90 were capable of little or no ambulation even with assistance (26 GMFCS level IV and 64 GMFCS level V). Weight z score proved to be the best predictor of BMD z score. Declining BMD z scores also correlated with increasing age and greater severity of involvement. It can be predicted, with reasonable reliability, that a 10-year-old non-ambulatory child with quadriplegic CP and a ‘typical’ weight z score of –2 will have a BMD z score that is at best –2. Prior fractures, use of anticonvulsants, and feeding difficulties further reduce predicted BMD.