Abstract. Let W be an operator weight, i.e. a weight function taking values in the bounded linear operators on a Hilbert space H. We prove that if the dyadic martingale transforms are uniformly bounded on L 2 R (W ) for each dyadic grid in R, then the Hilbert transform is bounded on L 2 R (W ) as well, thus providing an analogue of Burkholder's theorem for operator-weighted L 2 -spaces. We also give a short new proof of Burkholder's theorem itself. Our proof is based on the decomposition of the Hilbert transform into "dyadic shifts".