The present paper analyzed the association between children's sleeping positions and dental arch morphology. The sleeping patterns of 27 children, aged 3 to 10 yr, suspected of having the obstructive sleep apnea syndrome (OSAS) were studied by polysomnography (PSG) and videotape recordings under laboratory conditions. The PSG recordings were used to calculate the apnea index (AI) and the relative time spent sleeping on the back, and the videotapes to categorize distinctly different sleep and head postures. Plaster casts were made for the assessment of dental arch morphology. Sleeping predominantly on one's back, was associated with a reduced maxillary intercanine width, while prolonged head extension during sleep correlated inversely with the overjet. The subjects with the highest AI scores (> 4) had larger dental arches. We suggest that sleeping on the back causes a more posterior tongue position, reducing its moulding effect on the anterior dental arch. As nasopharyngeal airway obstruction in OSAS-patients might trigger an anterior tongue position to secure a free airway passage, there will be increased lingual pressure on the dental arches, leading to their dimensional increase.