Cybersickness is often experienced when viewing virtual environments through head-mounted displays (HMDs). This study examined whether vection (i.e., illusory self-motion) and mismatches between perceived and physical head motions contribute to such adverse experiences. Observers made oscillatory yaw head rotations while viewing stereoscopic optic flow through an Oculus Rift HMD. Vection and cybersickness were measured under 3 conditions of visual compensation for physical head movements: "compensated", "uncompensated", and "inversely compensated". When a nearer aperture was simulated by the HMD, vection was found to be strongest in the "compensated" condition and weakest in the "inversely compensated" condition. However, vection was similar for all 3 conditions during full-field exposures. Cybersickness was most severe for the "inversely compensated" condition, but was not different for the other two conditions. We conclude that mismatches between perceived and physical head-movements can contribute strongly to cybersickness. The relationship between vection and cybersickness is weaker and appears complex.