Traumatic subdural hygroma (TSH) is frequently bilateral and locates on the top of the head in a supine position. It suggests that the gravity and cranial posture act a certain role. The authors tried to test this hypothesis. The computed tomographic (CT) scans or magnetic resonance (MR) images of 86 consecutive patients with TSH were re-evaluated. The symmetry of the cranium, the posture of the head during the radiological examinations, and the location of the lesion were all checked. The cranium was symmetrical in 47 patients and asymmetrical in 39 patients. TSH was more commonly bilateral in patients with symmetrical cranium than those with asymmetrical cranium (77% vs 62%). The asymmetrical cranium tended to turn to the flat side. It was more frequently oblique in MR images, which has a long scanning time, than in CT (29% vs 18%). In 39 asymmetric craniums, TSH was bilateral and it was symmetrical in 14 cases. In the remaining 25 cases, TSH located opposite to the flat side in 18 cases. In seven patients with the same side TSHs, four patients had it on the side of atrophy, two on the opposite side of a mass lesion. The gravity and cranial posture can predict the location of TSH. TSH usually occurs at the least pressure in the cranium as a lesion of ex vacuo.