Although hot, cold, and high pressure denaturation are well characterized, the possibility of negative pressure unfolding has received much less attention. Proteins under negative pressure, however, are important in applications such as medical ultrasound, and the survival of biopoloymers in the xylem and adjacent parenchyma cells of vascular plants. In addition, negative pressure unfolding is fundamentally important in obtaining a complete understanding of protein stability and naturally complements previous studies of high pressure denaturation. We use extensive replica-exchange molecular dynamics (REMD) simulations and thermodynamic analysis to obtain folding/unfolding equilibrium phase diagrams for the rniniprotein trp-cage (alpha-structure, 20-residue), the GB1 beta-hairpin (beta-structure, 16-residue), and the AK16 peptide (alpha-helix, 16-residue). Although the tip-cage is destabilized by negative pressure, the GB1 beta-hairpin and AK16 peptide are stabilized by this condition.