In this paper we discuss some of the details of the calculation of the macroscopic polarization, via the Berry-phase approach, using a general (numerical) local combination of atomic orbitals (LCAO) as a basis set. Previous implementations with a LCAO basis relied on the use of Gaussian expansions, where analytical formulas exist for the crucial matrix elements. However, our approach, which only requires the matrix elements of the position operator, can be more easily implemented in the case of numerical orbitals. This work is a necessary first step towards our main goal of applying the SIESTA code to the study of ferroelectric alloys. This code, which uses a numerical LCAO basis set, has been highly optimized for the treatment of large systems. We are confident that it will allow us to treat larger systems than the standard plane-waves methods, while keeping a reasonable accuracy. Test results for representative ferroelectric perovskites are presented. ©2000 American Institute of Physics.