The aim of this cross-sectional study was to investigate the frequency of decreased areal bone mineral density (aBMD) among patients with cerebral palsy (CP), as estimated by using various aBMD Z-score adjustment methods. In addition, this study examined factors related to decreased aBMD scores. One hundred and two children between the ages of 3.2 and 17.8 years were examined. In patients with severe CP, the incidences of decreased aBMD according to various adjusting methods based on decimal age, bone age, height age, and height-for-age Z-score (HAZ) were 79.5%, 69.5%, 51.9%, and 38.3%, respectively. Abnormal levels of calcium, phosphorus, alkaline phosphatase, parathyroid hormone, or anticonvulsant were not predictive for a decreased aBMD. Mean aBMD Z-scores were significantly lower in all aBMD Z-score adjustment methods in patients with severe CP compared to patients with mild-to-moderate CP, except for the adjustment method based on HAZ.