In a sample of orthodontic patients (27 boys and 25 girls, 10 to 14 years old), natural head position (NHP) was photographically recorded. A vertical axis was recorded on the photographs with a plumb line and transferred to the lateral head radiographs of the patients. Three cephalometric reference lines (sella-nasion, basion-nasion, and porion-orbitale) were compared with regard to their inclination to the horizontal plane. These angles showed standard deviations of between 4.5 degrees and 5.6 degrees. Reproducibility of NHP, assessed as the error of a single observation, was close to 2 degrees. These findings, in combination with the fact that the NHP represents a realistic appearance of patients, support its use as a basis for cephalometric analysis of dentofacial anomalies. A strong correlation was found between the inclinations of the three cephalometric reference lines to the horizontal plane. The close anatomic relationship between these lines, in combination with the small random movements of the head around NHP, were believed to explain the covariation.