Desorption mass spectrometry has undergone significant improvements since the original experiments were performed more than 90 years ago(1). The most dramatic change occurred in the early 1980s with the introduction of an organic matrix(2-4) to transfer energy to the analyte. This reduces ion fragmentation but also introduces background ions from the matrix. Here we describe a matrix-free strategy for biomolecular mass spectrometry based on pulsed-laser desorption-ionization from a porous silicon(5) surface. Our method uses porous silicon to trap analytes deposited on the surface, and laser irradiation to vaporize and ionize them. We show that the method works at femtomole and attomole levels of analyte, and induces little or no fragmentation, in contrast to what is typically observed with other such approaches(6-11). The ability to perform these measurements without a matrix(3,4,12,13) also makes it more amenable to small-molecule analysis. Chemical(14) and structural(15) modification of the porous silicon has enabled optimization of the ionization characteristics of the surface. Our technique offers good sensitivity as well as compatibility with silicon-based microfluidics and microchip technologies.