Abstract. We consider two-weight estimates for singular integral operators and their commutators with bounded mean oscillation functions. Hörmander type conditions in the scale of Orlicz spaces are assumed on the kernels. We prove weighted weak-type estimates for pairs of weights (u, Su) where u is an arbitrary nonnegative function and S is a maximal operator depending on the smoothness of the kernel. We also obtain sufficient conditions on a pair of weights (u, v) for the operators to be bounded from L p (v) to L p,∞ (u). One-sided singular integrals, like the differential transform operator, are considered as well. We also provide applications to Fourier multipliers and homogeneous singular integrals.1. Introduction. The Calderón-Zygmund decomposition is a powerful tool in harmonic analysis. Since its discovery in [6], many authors have used it to derive boundedness properties of singular integral operators. For instance, using the fact that the Hilbert and Riesz transforms are bounded on L 2 , and by means of the Calderón-Zygmund decomposition, one proves that these classical operators are of weak type (1, 1). From this starting point, in the literature one can find many boundedness results for the Hilbert and Riesz transforms: estimates on L p , one-weight and two-weight norm inequalities, etc.The Calderón-Zygmund theory generalizes these ideas to provide a general framework allowing one to deal with singular integral operators. A typical Calderón-Zygmund convolution operator T is bounded on L 2 (R n ) and has a kernel K on which various conditions are assumed. In the easiest case, K behaves as the kernel of the Hilbert or Riesz transform. That is, K decays as |x| −n and its gradient as |x| −n−1 . It was already proved in [15] that these assumptions can be relaxed if we want to show that T is of weak type (1, 1):