For the first time, the gas pressure of crack-shaped nanocavities formed in silicon upon implantation with helium and subsequent annealing has been directly quantified from diffraction contrast features visible in transmission electron micrographs taken under dynamical two-beam conditions. For this purpose simulated images, based on the elastic displacements associated with a Griffith crack, have been matched to experimental micrographs thus yielding unambiguous data on the ratio plu of the cavity gas pressure to the silicon matrix shear modulus. Experimental results demonstrate cavity radii of some ten nanometres and p/,u values up to 0.22, which may be regarded as sufficiently high for the emission of dislocation loops from the nanocavities.