We present quantum mechanical calculations using density functional theory and semiempirical methods, and molecular mechanics (MM) calculations with a Tersoff-Brenner potential that explore the role of vacancy defects in the fracture of carbon nanotubes under axial tension. These methods show reasonable agreement, although the MM scheme systematically underestimates fracture strengths. One- and two-atom vacancy defects are observed to reduce failure stresses by as much as similar to26% and markedly reduce failure strains. Large holes - such as might be introduced via oxidative purification processes -greatly reduce strength, and this provides an explanation for the extant theoretical-experimental discrepancies. (C) 2004 Published by Elsevier B.V.