Fifteen patients aged 10 to 18 years with syncope of unknown origin, and 10 healthy control children aged 11 to 18 years, were evaluated by head-up tilt to 60 degrees for 60 minutes. Six patients (43%) reproduced symptoms of syncope during the examination. Four had a typical vasovagal reaction; two had marked hyperventilation. None of the children in the control group had syncope. The head-up tilt test offers a simple, noninvasive, high-yielding diagnostic tool for evaluation of syncope in children.