We consider a higher dimensional version of the Benjamin-Ono equation, ∂tu−R 1 ∆u+u∂x 1 u = 0, where R 1 denotes the Riesz transform with respect to the first coordinate. We first establish sharp space-time estimates for the associated linear equation. These estimates enable us to show that the initial value problem for the nonlinear equation is locally well-posed in L 2 -Sobolev spaces H s (R d ), with s > 5/3 if d = 2 and s > d/2 + 1/2 if d 3. We also provide ill-posedness results.With d = 1, the available local well-posedness theory has been based on compactness methods. Indeed, Molinet, Saut and Tzvetkov [19] proved that the problem cannot be solved in L 2 -Sobolev spaces H s by Picard iteration. We will show that