We recorded the responses of 189 medial superior temporal area (MST) neurons by using optic flow, real translational movement, and combined stimuli in which matching directions of optic flow and real translational movement were presented together. One-half of the neurons (48%) showed strong responses to optic flow simulating self-movement in the horizontal plane, and 24% showed strong responses to translational movement. Combining optic flow stimuli with matching directions of translational movement caused substantial changes in both the amplitude of the best responses (44% of neurons) and the strength of direction selectivity (71% of neurons), with little effect on which stimulus direction was preferred. However, combining optic flow and translational movement such that opposite directions were presented together changed the preferred direction in 45% of the neurons with substantial changes in the strength of direction selectivity. These studies suggest that MST neurons combine visual and vestibular signals to enhance self-movement detection and disambiguate optic flow that results from either self-movement or the movement of large objects near the observer.